feat: add parsers module and better match iter

The `parsers` module manages parsers for us, for now only in a really
basic way.

iter_prepared_mathes iters on an enhanced versions of the matches, where
captures are directly accessible via their names to allow things like :
    ((itentifier) @def.first (identifier) @def.last)
To be handled like this in lua:
    match.def.first
    match.def.last

Also adds a `set!` predicate to allow setting data within the prepared
match (see queries/lua/locals.scm) for examples.

lua/nvim-treesitter/parsers.lua

@@ -0,0 +1,21 @@
+local api = vim.api
+local ts = vim.treesitter
+
+local M = {}
+
+function M.has_parser(lang)
+  local lang = lang or api.nvim_buf_get_option(0, 'filetype')
+  return #api.nvim_get_runtime_file('parser/' .. lang .. '.*', false) > 0
+end
+
+function M.get_parser(bufnr)
+  if M.has_parser() then
+    local buf = bufnr or api.nvim_get_current_buf()
+    if not M[buf] then
+      M[buf] = ts.get_parser(buf)
+    end
+    return M[buf]
+  end
+end
+
+return M