Allow non-constant string message for assert

[Spartan322]

Fixes #47157
Based on #47165 reimplemented with modifications suggested from #47165 (comment)

Removes constant string requirement for assert message, analyzer does not account for some dynamic assignment.

Original Behavior

assert(false, "Constant String")

var _testDynamicString = "test"
assert(false, _testDynamicString) # Errors: Expected constant string for assert error message

var _testExplicitString : String = "test"
assert(false, _testExplicitString) # Errors: Expected constant string for assert error message

var _testImplicitString := "test"
assert(false, _testImplicitString) # Errors: Expected constant string for assert error message

var _testExplicitHint : int = 1
assert(false, _testExplicitHint) # Errors: Expected constant string for assert error message

var _testImplicitHint := 1
assert(false, _testImplicitHint) # Errors: Expected constant string for assert error message

var _testDynamicAssign = 1
assert(false, _testDynamicAssign) # Errors: Expected constant string for assert error message
_testDynamicAssign = "test"
assert(false, _testDynamicAssign) # Errors: Expected constant string for assert error message
_testDynamicAssign = 1
assert(false, _testDynamicAssign) # Errors: Expected constant string for assert error message

New Behavior

assert(false, "Constant String")

var _testDynamicString = "test"
assert(false, _testDynamicString)

var _testExplicitString : String = "test"
assert(false, _testExplicitString)

var _testImplicitString := "test"
assert(false, _testImplicitString)

var _testExplicitHint : int = 1
assert(false, _testExplicitHint) # Errors: Expected string for assert error message

var _testImplicitHint := 1
assert(false, _testImplicitHint) # Errors: Expected string for assert error message

var _testDynamicAssign = 1
assert(false, _testDynamicAssign) # Errors: Expected string for assert error message
_testDynamicAssign = "test"
assert(false, _testDynamicAssign)
_testDynamicAssign = 1
assert(false, _testDynamicAssign)

Potential Considerations

Since there is the capacity for non-string assert messages to escape the error here, as was suggested at #47157 (comment) a warning may be suitable in place of the original error. I was unsure whether that was desirable to put here or not so I have left any attempt out.

[KoBeWi]

Seems like this will happily accept non-string values and use them as a message. Maybe it should result in runtime error?
Although if it works then no reason to block it I guess 🤔

The PR fixes the issue, so it would be nice to get it merged. It blocks porting of my plugin to 4.0 >_>

[Chaosus]

It works okay, but the error should be emitted for the following statement

	var _testDynamicAssign = "TEST"
	_testDynamicAssign = 1
	assert(false, _testDynamicAssign) # Prints Assertion failed: 1

the type retrieved by _reduce_expression is incorrect, but the fixing may be difficult, so I would approve it if the fix is urgent since it correct (but incomplete).

[TokisanGames]

Most of us are using formatted strings in asserts and aren't so concerned about an error from printing a number. Many of us are also static typing, so will never encounter this. I have all of my asserts commented out and replaced with dummy constant strings. It would be nice to merge this so I can undo all of those hacks and get informative asserts sooner rather than later.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉