fix(ts): correctly highlight table headings

[PriceHiller]

Summary

Ensures a org table only has its heading get the @org.table.heading highlight/group applied.

Prior to this, it seems the relevant highlight query accidentally applied to the body of tables as well.

Before this change:

After this change:

Changes

• Updates the TS query for table heading highlights to restrict them to only the first heading

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Thanks!