define a js_str macro for creating JSONText

[hhaensel]

I propose to define a js_str macro to generate JSONText with and without interpolation as below.

Besides a handy short form a big advantage is that in VSCode JS-Syntax-Highlighting can be achieved by triple-quoting, e.g.

js"""
console.log('This looks pretty!')
"""

Many packages that handle js expressions, e.g. GenieFramework or Electron, would benefit (davidanthoff/Electron.jl#154) (GenieFramework currently exports its own version)

js"""alert("Hello World")"""
# JSONText("alert(\"Hello World\")")

Interpolation could be supported via the i flag, such as

js"""alert("1 + 2 == $(1 + 2)")"""i
# JSONText("alert(\"1 + 2 == 3\")")

I'll happily file a PR.

[hhaensel]

I decided to add this definition to my existing #427, I hope that's fine.
I also removed the JSONText definition for Symbols as you mentioned that you didn't like that.