Allow Unicode characters in GDShader identifiers

[laomengwa]

I want to take a Chinese character as a variable identifier in GDShader, but, use Chinese character variable is invalid.

And also, I use other language letter such as "Ö", "の" also invalid too.

At the moment, GDShader only use ASCII code identifiers.It was frustrating for me, who was not familiar with English.

In GDScript already allow use unicode characters ( GDScript Can use Chinese character variable identifier. ), I hope GDShader allow use unicode characters too.

Refer to other issue: #916

[AThousandShips]

Does glsl support this? GDShader is just converted to glsl so we can't support things that it doesn't support (unless we somehow translate it into a format that glsl supports)