- move credits and backers to credits.txt

Co-authored-by: David Pedersen <limero@me.com>
Co-authored-by: Gregory Anders <greg@gpanders.com>
Co-authored-by: Leo Schlosser <Leo.Schlosser@Student.HTW-Berlin.de>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.

Problem:
In h2 headings, the first tag points to an invalid anchor. This used to
work but regressed a few months ago, possibly related to
ceea6898a8bdcb6c4cfe06b8dc4739c144e6b1f8.

Solution:
- Simplify the logic, don't try to be clever:
  - Always use to_heading_tag() for the h2 `id`.
- Also:
  - Render tags as `<span>`, because `<code>` is unnecessary and doesn't
    look great in headings.
  - In the main h1, use "foo.txt" as the anchor `name` (rarely used),
    prefer the next found tag for the `href`.

Problem:
The <br> hack in a0c64fe816f8 causes weird layout if a "h4 pseudo-heading"
is not the only tag on the line. For example in the help text below, the
"*'buflisted'*" tag was treated as h4 and followed by <br>, which is
obviously wrong:

                            *'buflisted'* *'bl'* *'nobuflisted'* *'nobl'* *E85*
    'buflisted' 'bl'    boolean (default on)

Solution:
Only treat a tag as "h4 pseudo-heading" if it is the only tag in the
line. This is fragile, but in practice seems to get the right balance.

Problem:
The right-aligned tag "pseudo-heading" layout mushes together with the
left-aligned text. This is especially messy in a narrow viewport.

Solution:
Put a `<br>` on it. This is a hack until tree-sitter-vimdoc recognizes
these pseudo-headings.

fix #25401