Resolve issue caddyserver#7557

So, here is the situation:
- Pull request caddyserver#7206 included some changes to the doImport's function of
  Caddyfile's parser. What it does is that if there is no token within a
  block that follows the import, and the import contains `{block}`, then
  the `{block}` token is discarded.
- This implementation had a few flaws:
  - Issue caddyserver#7518 noticed that in cases that `{block}` was not imported,
    a runtime error was raised due to the assumption that tokens were
    always added to `tokensCopy` on every iteration of `importedTokens`.
    This was fixed by pull request caddyserver#7543.
  - Issue caddyserver#7557 notices that {block} can be ignored when imported from a
    certain file. There, it's again an issue with how the import works.
    When `import snippets` is called, this import instruction doesn't
    contains any nested blocks. And when the argument replacer that is
    the `importedTokens` loop is called and finds `{block}`, it uses the
    block from the file's import (which in this case is nothing),
    `{block}` is erased, and unavailable when the import directive is
    called for the imported snippet.

These changes fix the issues raised in caddyserver#7557 and tries to stabilize
the situation by adding a `Disposable` field to `Token`. It works like
this:
- When a `{block}` is encountered in the argument replacer loop, instead
  of ignoring it and not copying it, the token is now copied (instead of
  being erased), but with the `Disposable` field toggled on.
- When subsequent imports happens, the "disposable" `{block}` token is
  available for the processing of the new import, and can be used to
  call snippets from other files.
- When all tokens are processed within a server block, disposable tokens
  are removed from the keys and segments of the blocks.

This way, `{block}` tokens are there for subsequent inputs, and are
removed if unused.

Tests added in pull requests caddyserver#7206 and caddyserver#7543 passes with this new
implementation, confirming that unused `{block}` are accepted if nothing
is passed to `import`, as well as the other usual tests.
A new test was also added based on issue caddyserver#7557 reporting, and also passes.

... hopefully this commit description is clear?

Signed-off-by: prettysunflower <me@prettysunflower.moe>

Resolve issue caddyserver#7557

So, here is the situation:
- Pull request caddyserver#7206 included some changes to the doImport's function of
  Caddyfile's parser. What it does is that if there is no token within a
  block that follows the import, and the import contains `{block}`, then
  the `{block}` token is discarded.
- After this pull request:
  - Issue caddyserver#7518 noticed that in cases that `{block}` was not imported,
    a runtime error was raised due to the assumption that tokens were
    always added to `tokensCopy` on every iteration of `importedTokens`.
    This was fixed by pull request caddyserver#7543.
  - Issue caddyserver#7557 notices that {block} can be ignored when imported from a
    certain file. There, it's again an issue with how the import works.
    When `import snippets` is called, this import instruction doesn't
    contains any nested blocks. And when the argument replacer that is
    the `importedTokens` loop is called and finds `{block}`, it uses the
    block from the file's import (which in this case is nothing),
    `{block}` is erased, and unavailable when the import directive is
    called for the imported snippet.

The changed in this commit addresses the second issue by checking before
replacing `{block}` if we're currently in a snippet definition, and
appending the `{block}` token to `tokensCopy` if we are.

With this changes, when importing those snippets, the `{block}` token
will be available to be replaced by the nested blocks in `tokensToAdd`
if needed, or erased if there are no nested blocks and `tokensToAdd` is empty.

Tests added in pull requests caddyserver#7206 and caddyserver#7543 passes with this new
implementation, confirming that unused `{block}` are accepted if nothing
is passed to `import`, as well as the other usual tests.
A new test was also added based on issue caddyserver#7557 reporting, and also passes.

Signed-off-by: prettysunflower <me@prettysunflower.moe>