add rg support and ensure line number order in xref buffer.

the default search procedure is still ag. If you want to use rg
include `(setq xref-js2-search-program 'rg)` in your config file
or set `xref-js2-search-program` to `'rg` via the customize menu.

with xref-js2, ripgrep is configured to duplicate the operation
of ag. search queries are case insensitive by default. you can
customize the arguments passed to rg by setting `xref-js2-rg-arguments`
to your desired values. filtering based on file extensions is also
implemented, but unlike ags built in '--js' flag, you must manually
specify conforming extensions (or really just any valid suffix) in
`xref-js2-js-extensions`.

NOTE: `xref-js2-ignored-dirs` & `xref-js2-ignored-files` have been
      kept with their initial ag compatible values and work is done
      at runtime to make them compatible with ripgrep. If you wish
      to add more values to these variables, use ag compatible values.

Add regexp for es6 function definition

Nits And Projectile

Also try to find the current root-dir with projectile if available.

Nice if you're already using projectile but have turned off vc-mode
backends for example.

This makes it easier to find the package source code when browsing in
MELPA.

Ignore Emacs backup files, compiled files and TAGS.

Autoload xref-js2-xref-backend

This allows the setup instructions to work without needing to load xref-j2 first.