GDScript: support variable declarations as condition for if-statement

[zjin123]

Implement godotengine/godot-proposals#2727 for if-statement.

This implementation is simple and intuitive but not elegant.

Idea is to parse code like

if var a := foo():
    do_something(a)
else:
    do_other_thing()

if a > b, var c := bar(a, b), c > d:
    do_something(c)
else:
    do_other_thing()

into code llike

if true:
    var a := foo()
    if a:                           # jump address when !a will be patched to
        do_something(a)
else:                               # ... here
    do_other_thing()

if true:
    if a > b:                       # all of jump address when !(a>b)
        var c := bar(a, b)
        if c:                       # ... and jump address when !c
            if c > d:               # ... and jump address when !(c>d)
                do_something(c)
else:                               # ... will be patched to here
    do_other_thing()

Such transform will guarantee short-circuit evaluation and scope visibility.

Semantics will be ensured by patching jump addresses (as above) duration compilation.

[zjin123]

It appears generating IfNode for each condition breaks warning annotation function for all if-statement. This implementation may not be suitable.