Add support for Unicode identifiers in GDScript

This is using an adapted version of UAX#31 to not rely on the ICU
database (which isn't available in builds without TextServerAdvanced).
It allows most characters used in diverse scripts but not everything.

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

@@ -0,0 +1,35 @@
+const π = PI
+var ㄥ = π
+
+func test():
+	var փորձարկում = "test"
+	prints("փորձարկում", փորձարկում)
+	var امتحان = "test"
+	prints("امتحان", امتحان)
+	var পরীক্ষা = "test"
+	prints("পরীক্ষা", পরীক্ষা)
+	var тест = "test"
+	prints("тест", тест)
+	var जाँच = "test"
+	prints("जाँच", जाँच)
+	var 기준 = "test"
+	prints("기준", 기준)
+	var 测试 = "test"
+	prints("测试", 测试)
+	var テスト = "test"
+	prints("テスト", テスト)
+	var 試験 = "test"
+	prints("試験", 試験)
+	var പരീക്ഷ = "test"
+	prints("പരീക്ഷ", പരീക്ഷ)
+	var ทดสอบ = "test"
+	prints("ทดสอบ", ทดสอบ)
+	var δοκιμή = "test"
+	prints("δοκιμή", δοκιμή)
+
+	const d = 1.1
+	_process(d)
+	print(is_equal_approx(ㄥ, PI + (d * PI)))
+
+func _process(Δ: float) -> void:
+	ㄥ += Δ * π