Add exporting to native typescript enums with string values

[xazukx]

Goal

Add native typescript enums with string values. Integer values are not supported.
I have been using this in a very large codebase with many enum types (simple enums, tuple children, struct children, etc) for a few months with no problems. I've decided to open this PR and bring it to the world.

Changes

I added the attribute #[ts(use_ts_enum)] in order to turn a simple rust enum into a typescript enum. Case renaming still works except kebab renaming, because the name of the enum variant must be a valid typescript identifier. Proper warnings and errors are issued if invalid attributes are used alongside use_ts_enum on the same enum.
Inlining an enum in a struct switches back to using union types.

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

[stunningpixels]

This is much needed for many production use-cases. Would be great to see this merged.

[fbartho]

I too would find enums super helpful in my codebase! (I came to this PR to see how I could help!)

[NyxCode]

Hey, thanks for the PR! Sorry that it took so long for me to actually take a look at it.
@gustavo-shigueo has implemented some improvements of your PR in #425, supporting integer discriminants and all inflections. So I think it make sense to continue work on this feature over there.
Again, thanks for getting this work started!

Edit by @gustavo-shigueo: fix user name