refactor: parser list and lang->ft/ft->lang

- move parser list in `parsers.lua`
- most `ft` variable where changed to `lang`, `ft` is only used on
  autocmd binding, and lang is used for everything else. Functions have
  been defined to make the switch between `ft` and `lang`

lua/nvim-treesitter/health.lua

@@ -4,7 +4,7 @@ local fn = vim.fn
 local queries = require'nvim-treesitter.query'
 local locals = require'nvim-treesitter.locals'
 local highlight = require'nvim-treesitter.highlight'
-local configs = require'nvim-treesitter.configs'
+local parsers = require'nvim-treesitter.parsers'
 
 local health_start = vim.fn["health#report_start"]
 local health_ok = vim.fn['health#report_ok']
@@ -62,7 +62,7 @@ function M.checkhealth()
 
   local missing_parsers = {}
   -- Parser installation checks
-  for _, parser_name in pairs(configs.available_parsers()) do
+  for _, parser_name in pairs(parsers.available_parsers()) do
     local installed = #api.nvim_get_runtime_file('parser/'..parser_name..'.so', false)
 
     -- Only print informations about installed parsers