[FEATURE] Support text language and direction (lang/dir) in ReST

[linawolf]

Resolves #1266
Resolves #1267

Authors had no way to mark a block, a run of inline text, or a whole
page as being in a different language or text direction, so the
rendered HTML never got the lang/dir attributes needed for correct
rendering (e.g. RTL scripts, screen readers, correct font/glyph
shaping). The maintainers asked what concrete output this should
produce.

docutils/Sphinx were checked for prior art before designing this:

• Document level: neither has any per-document mechanism at all (only
  project-wide build config), so nothing to align with there.
• Inline level: docutils' own FAQ recommends exactly :rtl:/:ltr:
  roles for this, just via .. role:: boilerplate the author has to
  write themselves; this makes them built-in instead.
• Block level: docutils has .. class:: language-<tag> and
  .. class:: rtl/ltr, but that's CSS-only styling hooks with no
  real HTML lang/dir attribute, so it doesn't actually address the
  accessibility problem these issues are about. Kept our own
  :lang:/:dir: container options, which do emit real attributes.

This reuses existing mechanisms at three levels rather than inventing
new directives:

• Block-level: :lang:/:dir: options on the existing .. container::
  / .. div:: directive.
• Inline-level: new :rtl:/:ltr: roles for pure direction switches,
  and a combined :lang: role (:lang:text (language, direction)) for text tagged with both, following the same trailing-(...)convention as the existing:abbreviation:` role.
• Document-level: new :lang:/:dir: field-list metadata, mirroring
  the existing :template: field, setting the page's own <html> tag.

dir is validated against ltr/rtl/auto and lang against a loose
BCP 47 shape; both are warn-only diagnostics, never blocking, matching
the ValueType validation philosophy already used elsewhere.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PP4LkejR5PSubbhNmF4RkT

[jaapio]

I think direction should be an enum rather than an arbitrary string, this allows us to be more strict. We can validate the value before we create the enum, and otherwise fall back to auto when the value is invalid.

This will make sure rendering always works even if the userinput is wrong.

[linawolf]

@jaapio changes applied, please have another look