all: add support for enum Xyz as u64 { + tests (#16246)

doc/docs.md

@@ -3237,7 +3237,7 @@ You can see the complete
 ### Enums
 
 ```v
-enum Color {
+enum Color as u8 {
 	red
 	green
 	blue
@@ -3254,6 +3254,8 @@ match color {
 }
 ```
 
+The enum type can be any integer type, but can be ommited, if it is `int`: `enum Color {`.
+
 Enum match must be exhaustive or have an `else` branch.
 This ensures that if a new enum field is added, it's handled everywhere in the code.