Support external lexers

[dnbln]

Currently, all quoted_terminals yield &str as their type.

I played around with the grammar and lexer a little in https://github.com/dblanovschi/intellij-lalrpop/commit/9336ee964bb19832eb33c3e8851b638a324d2b0d, but didn't get to resolve quoted_terminal references yet and will have to research if descendentsOfType yields the same order as lalrpop.

[evbo]

Also tokens redefined in an extern are underlined red by this plugin that they do not exist with a message like:

Cannot resolve ___