add PossiblyReadAsNullable field value
- rename AllNodesPossiblyReadAsUndefined to ReplaceRemoveAndPossiblyReadAsNullableWithNever(internal)
- AllNodesPossiblyReadAsNullable union the type with null and undefined
- rename ReplaceRemoveWithUndefined with ReplaceRemoveAndPossiblyReadAsNullableWithNullable(internal)
- Removable now union with undefined and null
- Add new field value PossiblyReadAsNullable and its implementation and test
- update issue template
PossiblyReadAsNullable
is similar to Removable
in all aspects except only Removable grants the use of remove