* Improve completion in REPL
- refine `get_context_object` to avoid throwing errors (eg: q.<TAB>)
- do not call `eval` in `get_context_object` to avoid throwing errors
and reduce bloat caused by variable closures.
- support completion of directives
* Improve `Date.parse()`
- rewrite `Date.parse()` with separate parsers
- return `NaN` for out of bounds field values as specified
- add `js_tzabbr` and `string_get_tzabbr` to handle timezone abbreviations
- improve `string_get_milliseconds` readability
- accept up to 9 decimals for millisecond fraction but truncate at 3
- accept many more alternative date/time formats
- add test cases in **tests/test_builtin.js**
- produce readable output for `Date` objects in repl
- use `JSON.stringify` to output `Date` and `string` values in **repl.js**
- remove `String.prototype.__quote`
- add `minimum_length` macro to specify argument array sizes (C99 except MSVC)
- v8.js: parse all environment variables and output them, update **v8.txt**
