Adds pattern matching to GDScript

[karroffel]

As I proposed in #6593 here is the implementation of pattern matching in GDScript.

Right now the highlighting for the match keywords doesn't work but I think this isn't too important for code reviewing.

[karroffel]

Here the final "specification" of the pattern matching features.

---

The match statement can be used inside blocks (so where all the other statements can be used too).

Abstract syntax of a match statement:

match expression:
    pattern: block
    pattern: block
    ...

A pattern can be of 6 types:

• constant pattern
  • number|string|null .. any constant value.
    • Some expressions are foldable during parse time (like 21 * 2 or even abs(3)).
    • Note: function calls are not allowed if they cannot be reduced at parse time!
• variable pattern
  • identifier
    • matches against a variable/constant
• array pattern
  • [pattern, pattern, pattern, ... ]
    • trailing commas are allowed.
    • The last entry may be .. , which means "ignore the rest of the array".
    • All the element patterns are nestable (recursive)
• dictionary pattern
  • {constant: pattern, constant, ... }
    • trailing commas are allowed.
    • If no : is present, only the existence of the key is checked.
    • The last entry may be .. , which means "ignore the rest of the dictionary".
    • The sub-patterns are recursive as well.
• bind pattern
  • var identifier
    • binds the value that's being matched to the identifier.
• wildcard pattern
  • _
    • Ignores the value - always matches.

Edit: I forgot one, it isn't really a pattern type but anyway...

• multipattern
  • pattern, pattern, ... pattern
    • Checks for multiple patterns at once
    • comma separated patterns. Trailing comma is not allowed.
    • Bindings are not allowed in multipatterns
    • Matching is left-to-right

Since it's not really a pattern type it can't be used in array and dictionary patterns, only as a "toplevel" pattern.

The body can contain match statements as well.

---

Pattern are checked from top to bottom, left to right.

A break is not necessary, it's not even working (break is currently only allowed in loops). So it works when the match statement is inside of a loop, if it isn't a parse error will be thrown.
(It wouldn't be a big change to make it work like expected outside of loops, but I don't think that's a good idea)

A continue stops execution in the current block and checks the next pattern instead.

[karroffel]

If p_break_addr is changed to break_addr then break will work inside of a pattern branch. So no big changes needed to support that. However I think it would be a bad idea to do that since it will cause a lot of confusion amongst newbies and may lead people the wrong way.

break isn't match-idiomatic, so I think it would be a mistake to implement it that way.

[bojidar-bg]

Assigning to @reduz to review, since it is a new gdscript feature.

[pkowal1982]

I think GDScript syntax is complete, maybe missing ternary operator and while, so I'm against merging.

[karroffel]

@pkowal1982 ternary operators are implemented on master and while exists for a long time now. switch was a fairly often requested feature, this is just a one-up of a switch. See the issue #6593 for the discussion

[vnen]

🎉

[bojidar-bg]

@karroffel Now you have to write docs 😛

[karroffel]

oh snap. I actually thought about that in bed, will do after Akiens huge formatting code review

[bojidar-bg]

@karroffel I'm speaking of godotengine/godot-docs, heh