Fix is_valid_float, Variant parser, Expression parser, script highlighter, and TextServer not handing capital E in scientific notation.

modules/gdscript/tests/scripts/parser/features/constants.gd

@@ -4,6 +4,8 @@ func test():
 	const _VECTOR = Vector2(5, 6)
 	const _ARRAY = []
 	const _DICTIONARY = {"this": "dictionary"}
+	const _FLOAT1 = 1e2
+	const _FLOAT2 = 1E2
 
 	# Create user constants from built-in constants.
 	const _HELLO = PI + TAU