Update grammar to cover Flow syntax differences

[gkz]

A spreadsheet was created to document the different syntax between TypeScript, Flow, and some other type systems: https://docs.google.com/spreadsheets/d/1db9I5Yn495VIdOUFCKVbpoVlAyHlgW8fgyHwuXX4Ro4/edit#gid=1730464012

The grammar needs to be updated to take into account these differences. This would be in line with the commitment at the March 2022 TC39 meeting to make the proposal inclusive and not TypeScript specific.

I was asked to create this issue, so here it is!

TODO list:

• Maybe types ?T - https://flow.org/en/docs/types/maybe/
• Inline interface type type T = interface {}
• import typeof - https://flow.org/en/docs/types/modules/#toc-importing-and-exporting-values
• Variance on class fields class C {+field: string} - https://flow.org/en/docs/lang/variance/
• Opaque types opaque type T = ... - https://flow.org/en/docs/types/opaque-types/
• Optional indexed access T?.[K] - https://flow.org/en/docs/types/indexed-access/#toc-optional-indexed-access-types
• declare variants of things? e.g. to support function overloading - https://flow.org/en/docs/types/intersections/#declaring-overloaded-functions
• etc.

Some other differences are covered under the "token soup" as is currently proposed as far as I can see.

Can ignore for now:

• Explicit object types {| |} - will eventually be deprecated
• Casting syntax (x: T) - will be changing this