Convert SelectionDirection and SelectionMode in HTMLInputElement

[illusionalsagacity]

Our long term aim is to reduce the runtime code - when this library was written polymorphic variants were not strings. We can't convert existing modules until we do a wider breaking API change release in 2022, but for new modules we can make them as good as ReScript bindings allow.

So yes, please convert to polymorphic variants without runtime wrapping code.

Originally posted by @TheSpyder in https://github.com/tinymce/rescript-webapi/pull/73#discussion_r775669367

implementation should use polymorphic variants as described above

[TheSpyder]

There is a general tracking issue for this - #38