feat(treesitter): start moving get_parser to return nil #30313

**Problem:** `vim.treesitter.get_parser` will throw an error if no parser
can be found.

- This means the caller is responsible for wrapping it in a `pcall`,
  which is easy to forget
- It also makes it slightly harder to potentially memoize `get_parser`
  in the future
- It's a bit unintuitive since many other `get_*` style functions
  conventionally return `nil` if no object is found (e.g. `get_node`,
  `get_lang`, `query.get`, etc.)

**Solution:** Return `nil` if no parser can be found or created

- This requires a function signature change, and some new assertions in
  places where the parser will always (or should always) be found.
- This commit starts by making this change internally, since it is
  breaking. Eventually it will be rolled out to the public API.

runtime/lua/vim/_comment.lua

@@ -9,8 +9,8 @@
 local function get_commentstring(ref_position)
   local buf_cs = vim.bo.commentstring
 
-  local has_ts_parser, ts_parser = pcall(vim.treesitter.get_parser)
-  if not has_ts_parser then
+  local ts_parser = vim.treesitter._get_parser()
+  if not ts_parser then
     return buf_cs
   end