Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem:
`get_node_text()` returned inconsistent results between buffer and
string sources when a node's range ends at `end_col == 0` (i.e. the node
ends with a newline). The buffer path dropped the trailing newline; the
string path included it correctly.

Solution:
Append `'\n'` in `buf_range_get_text()` when `end_col == 0` and
`start_row ~= end_row`. The `start_row ~= end_row` guard excludes
zero-width nodes at column 0, which should return `""`.

Remove the workaround in the `#trim!` directive that manually
compensated for the missing newline.

Strip whitespace in `resolve_lang()` so injection language nodes ending
at `end_col == 0` (e.g. `">lua\n"`) still resolve correctly.

Test for regression #34605

Problem:
Indenting text is a common task in plugins/scripts for
presentation/formatting, yet vim has no way of doing it (especially
"dedent", and especially non-buffer text).

Solution:
Introduce `vim.text.indent()`. It sets the *exact* indentation because
that's a more difficult (and thus more useful) task than merely
"increasing the current indent" (which is somewhat easy with a `gsub()`
one-liner).

Breaking change: `ts_node_child_containing_descendant()` was removed

Breaking change: tree-sitter 0.25 (HEAD) required

**Problem:** `vim.treesitter.get_parser()` and `vim.treesitter.start()`
both parse the tree before returning it. This is problematic because if
this is a sync parse, it will stall the editor on large files. If it is
an async parse, the functions return stale trees.

**Solution:** Remove this parsing side effect and leave it to the user
to parse the returned trees, either synchronously or asynchronously.

This commit also marks `child_containing_descendant()` as deprecated
(per upstream's documentation), and uses `child_with_descendant()` in
its place. Minimum required tree-sitter version will now be `0.24`.