ReadOnlyMap

[hackwaly]

Is your feature request related to a problem? Please describe.
There's ReadOnlyArray, but no ReadOnlyMap. There's @immut/sorted_map/SortedMap, but it's second class (no literal syntax) compared to Map.

Describe the solution you'd like
Introduce ReadOnlyMap, a readonly version Map.

Additional context
Add any other context or screenshots about the feature request here.

• I do think that this should be part of the core library and should be delivered to all the MoonBit developers.

[peter-jerry-ye]

@immut/hashmap?

[peter-jerry-ye]

A workaround is:

///|
struct ReadonlyMap[K, V](Map[K, V])