Support Json5 features

[simonrouse9461]

JSON5 brings ECMAScript 5.1 features to JSON. Jsonnet supports most of the JSON5 features with a few exceptions:

• Strings may span multiple lines by escaping new line characters.
• Numbers may be hexadecimal.
• Numbers may have a leading or trailing decimal point.
• Numbers may be IEEE 754 positive infinity, negative infinity, and NaN.

This means that the following JSON5 file cannot be properly loaded as valid Jsonnet:

{
  lineBreaks: "Look, Mom! \
No \\n's!",
  hexadecimal: 0xdecaf,
  leadingDecimalPoint: .8675309, andTrailing: 8675309.,
  nan: NaN, infinity: Infinity,
}

These features would be great additions to Jsonnet. Supporting them will make Jsonnet a superset of JSON5 and thus make the migraion from JSON5 to Jsonnet much easier. Additionally, these features are mostly backward-compatible with existing Jsonnet syntax (except NaN and Infinity). Please consider supporting them.