Standardize new RegExp literal AST format

[mathiasbynens]

In Esprima, /foo/gi now gives:

{
  type: "Literal",
  value: /foo/gi,
  regex: {
    pattern: "foo",
    flags: "gi"
  }
}

The advantage of having the separate regex property is that even in environments that don’t support a given flag (e.g. the u flag) the regex can still be represented in the AST. The value property is null in such a case.

See https://code.google.com/p/esprima/issues/detail?id=557#c4.

cc @sebmck @michaelficarra

[Constellation]

And this style can be represented as JSON.

[sebmck]

This is also the behaviour of Acorn (acornjs/acorn@f48503c).

[nzakas]

Espree does the same