[Godot] Add pattern matching and string interpolation to GDScript's page

GDScript supports pattern matching in match statements:

```gdscript
match character:
var p when character is Character: print("Friend!")
var p when character is Enemy: print("Enemy!")
var p when character is NPC: print("Hi, i guess")
_: print("Nothing relevant")
```

But the page doesn't explain that. Also there is typed string interpolation and doesn't is in the page as well:

```gdscript
for i in 100:
print("%d is even?: %s" % [i, i % 2])
```

Open the repository's GDScript page and review the existing language-feature sections around match statements and strings. Add coverage for the two behaviors shown in the issue, keeping the examples and explanations consistent with the page; done means both features are documented clearly on that page.

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