GDScript: support variable definitions in if-statement

[zjin123]

Another attempt to implement godotengine/godot-proposals#2727 for if-statement. This patch implements two functions: allow variable definition as condition (if var n := foo():) and allow multiple conditions separated by comma in a single if-statement(if foo(), var n := bar(), n > 1:).

The ExpressionNode *condition field of IfNode is replaced with a list of either ExpressionNode or VariableNode. When parsing a single if-statement, a temporary block(SuiteNode) is created for parsing multiple conditions within and then the true block within. For an expression condition, one ExpressionNode is created. For a variable definition, one VariableNode and one ExpressionNode are created. The former one is used for generating code for initial assignment while the later one is used for testing as usual.

Compilation is done like usual except that, all of the not-true-jump-address for each condition will be patched to the same starting address else/elif if any, otherwise the end address of the if statement. Thus, the write_else() interface is changed to write_else(int count).

• Bugsquad edit, closes: Allow to define variables inside conditions godot-proposals#2727

[KoBeWi]

I'm not sure about the comma syntax, it's basically a weird and that allows multiple definitions. I think single definition is enough in most cases.