Prune/remove quoted verbatim strings

[JarrettBillingsley]

Now that the Lua-like block verbatim strings are in, the quoted verbatim strings are less useful. I originally had three kinds just so you could pick a different quote character from the string contents, but four kinds of verbatim strings seems a bit excessive now.

I do like the low visual impact of the quoted verbatim strings though.

Regex([=[\bhello\b]=])
Regex(@'\bhello\b')

The latter seems more readable to me than the former.

So maybe the backtick strings can be dropped and then there'll only be single- and double-quote strings, which makes a nice counterpart to single- and double-quoted regular strings.

[JarrettBillingsley]

Also, the "double-quote escaping" thing should probably go too. It's hard to explain, annoying to lex/highlight, and since there are block verbatim strings for longer text, not too useful.