Type fixes for time adapters

[joshkel]

Specific changes:

• formats is defined as an arbitrary record. In practice, it maps from TimeUnit and 'datetime' to the specific date library's format strings.
• parse and format were defined as requiring TimeUnit, but they actually take date library format strings. (E.g., it's up to the caller to look up format strings via formats() or user parameters.)
• endOf is never passed isoWeek (isoWeek isn't a normal TimeUnit, it's only used as a special case to startOf), and chartjs-adapter-date-fns doesn't support it.
• The constructor's options parameter is optional.
• weekday is documented as allowing a boolean (true means to start on Monday).
• export default { _date: DateAdapterBase } meant that new _date() returns a DateAdapterBase instance, whose methods take no parameters. Since it can be overridden at runtime, I replaced it with a more generic constructor + static methods.