Timezones not working for `fromMillis`

[loganvolkers]

It appears that the fromMillis is not respecting the timezone parameter. The docs say that it should default to UTC, but it appears to default to the system/browser timezone. Also you can not manually set the timezone.

Example JSONata expression: http://try.jsonata.org/rymaAsdZ8

[
 $fromMillis(1514808000000, 'Week: [w] of [xNn]'),
 $fromMillis(1514808000000, 'Week: [w] of [xNn]', "+0000"),
 $fromMillis(1514808000000, 'Week: [w] of [xNn]', "+0900")
]

Expected result

[
  "Week: 1 of January",
  "Week: 1 of January",
  "Week: 5 of December"
]

Actual result

[
  "Week: 5 of December",
  "Week: 5 of December",
  "Week: 5 of December"
]

This is causing tests the fail when they are run in other timezones, and likely causing incorrect results for anyone using the fromMillis function.

Also related to #347 and the lack of daylight-savings-time support.

aside I think that this should probably get patched, but also might want to direct people to using their system-native functions for formatting, like the Intl browser API and Luxon or moment.js. Datetime processing is notoriously difficult to get right.

[loganvolkers]

🥇 thanks for the quick fix!