fix(textobject): fall back to parent layers for tree-sitter textobjects

[beeb]

Closes #16366.

Since d27856b, textobject_treesitter only looks at the innermost injection layer under the cursor.
If that layer's language has no textobject query, or no matching capture, the selection does not change. It should fall back to the enclosing language.

The most visible case is comments: in Rust (and any language that injects comment), mic and mac do nothing when the cursor is inside a comment, because the comment language has no textobjects.scm.
Doc comments are affected the same way through the markdown-rustdoc injection.

The fix iterates through the layers at the cursor from inside to outside and uses the first one that gives a node containing the cursor.

Added a regression test that selects comment.inside with the cursor inside a Rust line comment.

Related: #16335 fixes the same problem for goto_treesitter_object (]c / [c, see #16333). The fixes are independent, though.