Backtick support

[koroshiya]

Adds support for backtick highlighting, auto-complete, etc. as an alternative to double or single quotes.

Backticks are useful for avoiding escape characters within lines of dialogue which contain both single and double quotes.
eg.

`So I said, "why can't I?"`

Rather than

"So I said, \"why can't I?\""

or

'So I said, "why can\'t I?"'

Renpy itself already plays nicely with backticks. It properly lints, compiles, extracts dialogue, generates translations, etc.

[williamd1k0]

Nice! (and sorry for the delay)

I tested it on Lightshow and looks nice.

Highlighting test: Lightshow

Unfortunately I don't have time now to properly review the regex :/

[koroshiya]

No problem. I realize it isn't something most people would use.
Regex should be fairly straightforward. I replicated what was done for single/double quotes using backticks, so the regex should be almost identical.