ReScript v11

[zth]

This tracks all the things we can/should do to fully utilize ReScript v11 in Core.

• Dedicated JSON unboxed variant type. Enable this pattern: https://rescript-lang.org/blog/improving-interop#decoding-and-encoding-json-idiomatically
• Null.t and Nullable.t should have an unboxed variant representation as well, so they can be pattern matched on.
• Ensure that Null.t and Nullable.t have all the convenience functions that Option currently has, so they're smooth to work with For later.
• Promise.allSettled bindings (cc @cometkim)

Please feel free to add your thoughts here. We'll fill in more things here as we go.

cc @cknitt @cristianoc

[youngkidwarrior]

I ran into this when upgrading to v11 as well in /src/Core__Error.res

The unsafe j\$( a )$( b )` interpolation was removed, use string template ${a}${b} instead.`

[zth]

This should be solved in 0.3.0, please try that version.