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/treesitter/_fold.lua

@@ -114,7 +114,7 @@ local function compute_folds_levels(bufnr, info, srow, erow, parse_injections)
   srow = srow or 0
   erow = erow or api.nvim_buf_line_count(bufnr)
 
-  local parser = ts.get_parser(bufnr)
+  local parser = assert(ts._get_parser(bufnr))
 
   parser:parse(parse_injections and { srow, erow } or nil)
 
@@ -392,7 +392,7 @@ function M.foldexpr(lnum)
   lnum = lnum or vim.v.lnum
   local bufnr = api.nvim_get_current_buf()
 
-  local parser = vim.F.npcall(ts.get_parser, bufnr)
+  local parser = ts._get_parser(bufnr)
   if not parser then
     return '0'
   end