refactor(treesitter): use scratch buffer for string parser #35988

This commit changes `languagetree.lua` so that it creates a scratch
buffer under the hood when dealing with string parsers. This will make
it much easier to just use extmarks whenever we need to track injection
trees in `languagetree.lua`. This also allows us to remove the
`treesitter.c` code for parsing a string directly.

Note that the string parser's scratch buffer has `set noeol nofixeol` so
that the parsed source exactly matches the passed in string.

test/functional/treesitter/parser_spec.lua

@@ -1264,6 +1264,12 @@ print()
       parser:for_each_tree(function(tstree, tree)
         ranges[tree:lang()] = { tstree:root():range(true) }
       end)
+
+      -- Scratch buffer should get cleaned up
+      assert(vim.api.nvim_buf_is_loaded(parser:source()))
+      parser:destroy()
+      assert(not vim.api.nvim_buf_is_loaded(parser:source()))
+
       return ranges
     end)