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`.

Problem:

Tests have lots of exec_lua calls which input blocks of code
provided as unformatted strings.

Solution:

Teach exec_lua how to handle functions.

Adds a new field `include_anonymous` to the `get_node` options to allow
anonymous nodes to be returned.

Problem: `has-ancestor?` is O(n²) for the depth of the tree since it iterates over each of the node's ancestors (bottom-up), and each ancestor takes O(n) time.
This happens because tree-sitter's nodes don't store their parent nodes, and the tree is searched (top-down) each time a new parent is requested.

Solution: Make use of new `ts_node_child_containing_descendant()` in tree-sitter v0.22.6 (which is now the minimum required version) to rewrite the `has-ancestor?` predicate in C to become O(n).

For a sample file, decreases the time taken by `has-ancestor?` from 360ms to 6ms.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.