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.lua

@@ -1,10 +1,13 @@
 local api = vim.api
+local parsers = require'nvim-treesitter.parsers'
 
 local M = {}
 
 -- This function sets up everythin needed for a given language
 -- this is the main interface through the plugin
 function M.setup(lang)
+  if parsers.has_parser(lang) then
+  end
 end
 
 return M