Calendar: Initial support for iCalendar format

[alec3660]

This PR adds basic support for loading iCalendar (.ics) files and parsing VEVENT properties that are currently supported by Calendar Events: summary, start and end.

[nico]

Cool!

[nico]

nit: Maybe it'd be nice to move ical parsing out into a separate ICalParser class? Doesn't have to be in this PR though.

[nico]

It's nice to add a few spec comments, like e.g.

    // 4.3.4 Date
    //   date               = date-value
    //   
    //   date-value         = date-fullyear date-month date-mday
    //   date-fullyear      = 4DIGIT
    //   date-month         = 2DIGIT        ;01-12
    //   date-mday          = 2DIGIT        ;01-28, 01-29, 01-30, 01-31
    //                                      ;based on month/year

    // 4.3.5   Date-Time
    //      date-time  = date "T" time ;As specified in the date and time
    //                                ;value definitions

    // 4.3.12 Time
    // time               = time-hour time-minute time-second [time-utc]
    // time-hour          = 2DIGIT        ;00-23
    // time-minute        = 2DIGIT        ;00-59
    // time-second        = 2DIGIT        ;00-60
    // ;The "60" value is used to account for "leap" seconds.
    // time-utc   = "Z"

…possibly interleaved with the code. We try to make code look a lot like the specs the code implement. (See e.g. GIFWriter.cpp or any file in LibJS for examples.)

4.3.5 mentions three forms (local time, utc, and explicit timezone). Supporting the first two seems pretty easy, might as well do that, and put in an explicit comment saying the third form isn't supported yet.