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.

test/functional/treesitter/language_spec.lua

@@ -8,6 +8,7 @@ local exec_lua = n.exec_lua
 local pcall_err = t.pcall_err
 local matches = t.matches
 local insert = n.insert
+local NIL = vim.NIL
 
 before_each(clear)
 
@@ -15,10 +16,12 @@ describe('treesitter language API', function()
   -- error tests not requiring a parser library
   it('handles missing language', function()
     eq(
-      ".../language.lua:0: no parser for 'borklang' language, see :help treesitter-parsers",
+      '.../treesitter.lua:0: Parser not found.',
       pcall_err(exec_lua, "parser = vim.treesitter.get_parser(0, 'borklang')")
     )
 
+    eq(NIL, exec_lua("return vim.treesitter._get_parser(0, 'borklang')"))
+
     -- actual message depends on platform
     matches(
       "Failed to load parser for language 'borklang': uv_dlopen: .+",
@@ -105,9 +108,10 @@ describe('treesitter language API', function()
       command('set filetype=borklang')
       -- Should throw an error when filetype changes to borklang
       eq(
-        ".../language.lua:0: no parser for 'borklang' language, see :help treesitter-parsers",
+        '.../treesitter.lua:0: Parser not found.',
         pcall_err(exec_lua, "new_parser = vim.treesitter.get_parser(0, 'borklang')")
       )
+      eq(NIL, exec_lua("return vim.treesitter._get_parser(0, 'borklang')"))
     end
   )