Remove luxon

[davidgoli]

Addresses #501

Note it is now possible to avoid using any 3rd party dependency, since browser support for the Intl API is now fairly widespread.

[codecov-commenter]

Codecov Report

Merging #508 (a6f6602) into master (17268f4) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.02%     
==========================================
  Files          29       29              
  Lines        2097     2094       -3     
  Branches      584      584              
==========================================
- Hits         1856     1853       -3     
  Misses        241      241              

Impacted Files	Coverage Δ
src/datewithzone.ts	100.00% <100.00%> (ø)

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[sunshineo]

If there are projects loading from these github.io urls, for example using a <script> tag in an html file, this change will break those projects. I recommend we make a copy of rrule.min.js and rrule.js to rrule-tz.min.js and rrule-tz.js and continue to place them under dist folder.
But on the other hand, I don't know can we push users of these 2 files to switch. If we do not want to keep provide these files forever, eventually we need to stop and whenever we stop, it will break some body, so why not break it now.

[davidgoli]

I hope nobody is linking directly to these files in a production app! It would be a very bad idea, since they're not versioned and can change unpredictably. Thus, I think it's a good idea to discourage that use case. These files are for direct download only. I would support removing even these links, and encourage everyone to just use npm as the source of truth for the package, but I think that's out of scope for this PR.

[sunshineo]

In README, there is still a reference to luxon and example
https://github.com/jakubroztocil/rrule/blob/remove-luxon/README.md?plain=1#L193

[davidgoli]

I chose to leave the Luxon reference in the README, since it's mentioned only as an example of a library that can be used to manipulate dates & time zones. Other packages eg. date-fns & day.js could just as easily work in place of Luxon to achieve the same result. I don't think it's necessary for this package's README to give examples in every possible Date library, so I think it's acceptable to leave a single example in an arbitrary package, in this case Luxon.

[cmmash]

So does this mean I can do this:
dtstart = 2010-01-01T10:00:00 (10am in my local tz)
tz = Asia/Kolkata
interval = 1
frequency = monthly

And the occurrences returned will be like this:
2010-01-01T10:00:00
2010-02-01T10:00:00
2010-03-01T10:00:00