Fix comment handling

[bemyak]

See the first comment for the gist of the changes.

With the current implementation, this is not parsed correctly:

@startuml
' Test comment
System(foo, "bar")
' Other comment

Everything between single quotes is treated as a string.

I tried to keep the changes minimal and backwards-compatible, but I'd consider combining string and single_quote_string into one directive: string: $ => choice(/"[^"]*?"/, /'[^']*?'/),.

Should resolve gabeins/zed-plantuml#5

[atassis]

Hi there. What stops us from merging this PR?