feat/refacto: add configs.lua, setup install

- configs.lua holds the `repositories` data
- install health moved to health.lua
- plugins loads _root.setup() on startup
- install and list command are available through vim
> use them with `:TSInstall lang` and `:TSInstallInfo`

lua/nvim-treesitter.lua

@@ -1,10 +1,15 @@
 local api = vim.api
 local parsers = require'nvim-treesitter.parsers'
+local configs = require 'nvim-treesitter.configs'
 local install = require'nvim-treesitter.install'
 local locals = require'nvim-treesitter.locals'
 
 local M = {}
 
+function M.available_parsers()
+  return vim.tbl_keys(configs.repositories)
+end
+
 -- This function sets up everythin needed for a given language
 -- this is the main interface through the plugin
 function M.setup(lang)
@@ -12,9 +17,11 @@ function M.setup(lang)
   end
 end
 
--- To install, run `:lua require'nvim-treesitter'.install_parser('language')`
--- we should add a vim layer over the lua function
-M.install_parser = install.install_parser
-M.list_parsers = install.list_parsers
+-- This function initialize the plugin
+-- it is run at startup
+M._root = {}
+function M._root.setup()
+  install.setup()
+end
 
 return M