By default, a plain-text label is used which does not support rich text
formatting.

Define a `Tooltip` scene; and override `Control._make_custom_tooltip()` in `Block` to
return an instance of this scene.

`Tooltip` is initially just a `RichTextLabel` with BBCode enabled and a custom
minimum size set as suggested by the `_make_custom_tooltip()` documentation:

> Note: The tooltip is shrunk to minimal size. If you want to ensure it's fully
> visible, you might want to set its `custom_minimum_size` to some non-zero
> value.

We override the theme styles on the `RichTextLabel` to match the styles used in
the built-in inspector's help tooltips. In particular, bold, italic and
bold-italic text doesn't work without overriding these styles. The [BBCode in
RichTextLabel][0] documentation claims:

> If no custom bold or italic fonts are defined, faux bold and italic fonts
> will be generated by Godot.

but empirically this is not true.

The Editor Theme Explorer plugin
<https://github.com/YuriSizov/godot-editor-theme-explorer> was useful, as was
reading the editor source code, specifically `editor_fonts.cpp` and
`editor_help.cpp`.

None of the existing `tooltip_text` properties in the codebase use the `[`
character that is used by BBCode tags, so there is no need to adjust the
existing tooltips to keep displaying as they previously did.

https://phabricator.endlessm.com/T35540

[0]: https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html#reference