This is a giant change for remark.
It replaces the 5+ year old internals with a new low-level parser:
<https://github.com/micromark/micromark>
The old internals have served billions of users well over the years, but
markdown has changed over that time.
micromark comes with 100% CommonMark (and GFM as an extension) compliance,
and (WIP) docs on parsing rules for how to tokenize markdown with a state
machine: <https://github.com/micromark/common-markup-state-machine>.
micromark, and micromark in remark, is a good base for the future.

`remark-parse` now defers its work to [`micromark`][micromark] and
[`mdast-util-from-markdown`][from-markdown].
`micromark` is a new, small, complete, and CommonMark compliant low-level
markdown parser.
`from-markdown` turns its tokens into the previously (and still) used syntax
tree: [mdast][].
Extensions to `remark-parse` work differently: they’re a two-part act.
See for example [`micromark-extension-footnote`][micromark-footnote] and
[`mdast-util-footnote`][from-markdown-footnote].

* change: `commonmark` is no longer an option — it’s the default
* move: `gfm` is no longer an option — moved to `remark-gfm`
* remove: `pedantic` is no longer an option — this legacy and buggy flavor of
  markdown is no longer widely used
* remove: `blocks` is no longer an options — it’s no longer suggested to
  change the internal list of HTML “block” tag names

remark-stringify now defers its work to [`mdast-util-to-markdown`][to-markdown].
It’s a new and better serializer with powerful features to ensure serialized
markdown represents the syntax tree (mdast), no matter what plugins do.
Extensions to it work differently: see for example
[`mdast-util-footnote`][to-markdown-footnote].

* change: `commonmark` is no longer an option, it’s the default
* change: `emphasis` now defaults to `*`
* change: `bullet` now defaults to `*`
* move: `gfm` is no longer an option — moved to `remark-gfm`
* move: `tableCellPadding` — moved to `remark-gfm`
* move: `tablePipeAlign` — moved to `remark-gfm`
* move: `stringLength` — moved to `remark-gfm`
* remove: `pedantic` is no longer an option — this legacy and buggy flavor of
  markdown is no longer widely used
* remove: `entities` is no longer an option — with CommonMark there is almost
  never a need to use character references, as character escapes are preferred
* new: `quote` — you can now prefer single quotes (`'`) over double quotes
  (`"`) in titles

All of these are for CommonMark compatibility.
Most of them are inconsequential.

* **notable**: references (as in, links `[text][id]` and images `![alt][id]`)
  are no longer present as such in the syntax tree if they don’t have a
  corresponding definition (`[id]: example.com`).
  The reason for this is that CommonMark requires `[text *emphasis
  start][undefined] emphasis end*` to be emphasis.
* **notable**: it is no longer possible to use two blank lines between two
  lists or a list and indented code.
  CommonMark prohibits it.
  For a solution, use an empty comment to end lists (`<!---->`)
* inconsequential: whitespace at the start and end of lines in paragraphs is
  now ignored
* inconsequential: `<mailto:foobarbaz>` are now correctly parsed, and the
  scheme is part of the tree
* inconsequential: indented code can now follow a block quote w/o blank line
* inconsequential: trailing indented blank lines after indented code are no
  longer part of that code
* inconsequential: character references and escapes are no longer present as
  separate text nodes
* inconsequential: character references which HTML allows but CommonMark
  doesn’t, such as `&copy` w/o the semicolon, are no longer recognized
* inconsequential: the `indent` field is no longer available on `position`
* fix: multiline setext headings
* fix: lazy lists
* fix: attention (emphasis, strong)
* fix: tabs
* fix: empty alt on images is now present as an empty string
* …plus a ton of other minor previous differences from CommonMark

* get folks to use this and report problems!

* make `remark-gfm`
* start making next branches for plugins
* get types into {from,to}-markdown and use them here

Closes GH-218.
Closes GH-306.
Closes GH-315.
Closes GH-324.
Closes GH-398.
Closes GH-402.
Closes GH-407.
Closes GH-439.
Closes GH-450.
Closes GH-459.
Closes GH-493.
Closes GH-494.
Closes GH-497.
Closes GH-504.
Closes GH-517.
Closes GH-521.
Closes GH-523.

Closes remarkjs/remark-lint#111.

[micromark]: https://github.com/micromark/micromark

[from-markdown]: https://github.com/syntax-tree/mdast-util-from-markdown

[to-markdown]: https://github.com/syntax-tree/mdast-util-to-markdown

[micromark-footnote]: https://github.com/micromark/micromark-extension-footnote/blob/main/index.js

[to-markdown-footnote]: https://github.com/syntax-tree/mdast-util-footnote/blob/main/to-markdown.js

[from-markdown-footnote]: https://github.com/syntax-tree/mdast-util-footnote/blob/main/from-markdown.js

[mdast]: https://github.com/syntax-tree/mdast