gdlint feature request: Checking enum variable assignment and operations

[heppocogne]

The enum type of GDScript behaves similarly to that of C/C++ language. That is, we can write some inappropriate operation which may lead bugs:

enum Example {
	VALUE_A,
	VALUE_B,
}

var example: Example
# CASE1: 10 is neither Example.VALUE_A nor Example.VALUE_B
example = 10
# explicit type conversion should be allowed
example = 20 as Examle

# CASE2: +, -, *, / operation usually gives meaningless result
var example2 = Example.VALUE_A + Example.VALUE_B
# use of &, |, ^ (and bit shift?) operator should be allowed
exmaple2 = Example.VALUE_A & Example.VALUE_B