Use lexical-binding in yasnippet.el

[eklitzke]

Emacs can optimize files that use lexical-binding slightly better, so this is a good thing to do. Additionally I've been experimenting with Andrea Corallo's feature/native-comp branch in upstream Emacs, and it can only native compile elisp files that use lexical-bidning, which is how I noticed that yasnippet.el did not.

The tests pass in this branch and I don't get any warnings when byte-compiling (or native compiling) the file with lexical-binding, so it seems like a safe change to make.

I agree to copyright assignment of this change as described in the contributing guidelines.

[npostavs]

Yeah, I've been holding off on this because yasnippet still supports 23.4, though it's probably coming time to drop that.

I agree to copyright assignment of this change as described in the contributing guidelines.

If you plan to contribute more patches to yasnippet or GNU Emacs, you can start the copyright assignment process by filling in this form, with "Emacs" as the program name, and send it to the mentioned email address.

It's not necessary for this patch, since it's obviously trivial.