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.

scripts/gen_help_html.lua

@@ -786,7 +786,7 @@ local function parse_buf(fname, parser_path)
   if parser_path then
     vim.treesitter.language.add('vimdoc', { path = parser_path })
   end
-  local lang_tree = vim.treesitter.get_parser(buf)
+  local lang_tree = assert(vim.treesitter._get_parser(buf), 'vimdoc parser not found.')
   return lang_tree, buf
 end