doc: add RegExp Unicode Property Escapes to intl

[vsemozhetbyt]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, intl

RegExp Unicode Property Escapes are at stage 4 and will be included in ES2018. They are available since V8 6.4 without a flag so they will be unflagged in Node.js v10. They are also available under the --harmony_regexp_property flag in Node.js v6-v9 and under the --harmony flag in Node.js v8-v9.

So it seems we should already warn about this and maybe backport these notes till v6 LTS.

Simple test:

> node.10.0.0.v8-6.4.20180227.nightly.exe

> process.versions

{ http_parser: '2.7.0',
  node: '10.0.0-nightly20180227a27e6d7321',
  v8: '6.4.388.46-node.4',
  uv: '1.19.2',
  zlib: '1.2.11',
  ares: '1.13.0',
  modules: '61',
  nghttp2: '1.29.0',
  napi: '2',
  openssl: '1.0.2n',
  icu: '60.2',
  unicode: '10.0',
  cldr: '32.0.1',
  tz: '2017c' }

> 'abc абв'.match(/\p{Letter}+/gu);

[ 'abc', 'абв' ]

'abc абв'.match(/\p{Script=Cyrillic}+/gu);

[ 'абв' ]

> .\vcbuild without-intl test

...

> node.10.0.0-pre.v8-6.4.20180228.without-intl.exe

> process.versions

{ http_parser: '2.7.0',
  node: '10.0.0-pre',
  v8: '6.4.388.46-node.4',
  uv: '1.19.2',
  zlib: '1.2.11',
  ares: '1.13.0',
  modules: '61',
  nghttp2: '1.29.0',
  napi: '2',
  openssl: '1.0.2n' }

> 'abc абв'.match(/\p{Letter}+/gu);

SyntaxError: Invalid regular expression: /\p{Letter}+/: Invalid property name

> 'abc абв'.match(/\p{Script=Cyrillic}+/gu);

SyntaxError: Invalid regular expression: /\p{Script=Cyrillic}+/: Invalid property name

I am not sure if this suffices to state full support also for system-icu, please check and correct me if this is needed.

Additional refs:

https://mathiasbynens.be/notes/es-unicode-property-escapes
https://mathiasbynens.be/notes/es-regexp-proposals#unicode-property-escapes

http://exploringjs.com/es2018-es2019/ch_regexp-unicode-property-escapes.html

https://ponyfoo.com/articles/regular-expressions-post-es6#unicode-property-escapes

Unfortunately, still no info in MDN RegExp page to link to.

[vsemozhetbyt]

CI-lite: https://ci.nodejs.org/job/node-test-pull-request-lite/202/ (both green)

[vsemozhetbyt]

Landed in d3af120