feat(setup)!: remove ensure_install field

Instead, call `require('nvim-treesitter').install( { ... } )` manually.

This gives users full control over how they want to install parsers
(sync, from grammar, limited concurrency) and obviates the need for
calling `setup` for most users.

lua/nvim-treesitter/init.lua

@@ -4,6 +4,18 @@ function M.setup(...)
   require('nvim-treesitter.config').setup(...)
 end
 
+function M.install(...)
+  require('nvim-treesitter.install').install(...)
+end
+
+function M.uninstall(...)
+  require('nvim-treesitter.install').uninstall(...)
+end
+
+function M.update(...)
+  require('nvim-treesitter.install').update(...)
+end
+
 function M.indentexpr()
   return require('nvim-treesitter.indent').get_indent(vim.v.lnum)
 end