Highlight JS templates

[ProgerXP]

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JavaScript has a relatively new syntax for strings:

let str = `line1
${eval} line2
${`nested ${1+1}`} line3`

I.e. like 'str' and "str" but multiline and allowing expression interpolation via ${...} (and nested templates via this construct).

Need to add highlighting for such strings. However, Scintilla probably has this already, maybe we should just update our schemes? Also see #180.

[cshnik]

Fixed.

[ProgerXP]

Was this implemented from scratch or it's using Scintilla rules? Because highlighting rules are not correct:

• backslash escapes are not correctly handled: foo backslash backtick bar is a single template because backtick is escaped but it's highlighted as foo as a template and bar as a code
• nested templates are not recognized - backtick before abc doesn't close the string, it's start of another string since anything inside ${ } is code, not part of the template

[cshnik]

Here are the points:

1. Scintilla's lexers are used when applying predefined highlighting rules.
2. CPP lexer is used for Javascript scheme.
3. Recent version of Scintilla still has no correct support of "JS template strings" feature: https://sourceforge.net/p/scintilla/feature-requests/1112/

[ProgerXP]

I see. Let's leave the rule enabled just in case later Scintilla devs get around to implementing proper template highlighting.