[AssemblyScript] Added basic AssemblyScript support

[LucasSwitz]

Couple notes that describe the differences from the TypeScript generator.

• Namespace
  • AS does not support dot-notation nested namespaces.
  • AS does not support namespace extensions
  • I have not implemented namespace support in this patch.
• Numbers:
  • Need to explicitly return i32 or f32 rather than number
  • Support is currently only added for 32 bit numbers. Adding 64 bit should be a simple extension.
• Return types:
  • void functions need to explicitly return void
• Unioned return types (a|b|c) are not supported currently.
• scalars are not nullable
• Cannot do (null | T ) need to explicitly check for null and assign manually.

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[LucasSwitz]

@googlebot I signed it!

[aardappel]

@krojew what do you think?

[LucasSwitz]

Just saw #6302 merged. Will likely require a good bit of massaging on my side to resolve conflicts.

[krojew]

I think this might warrant a new idl_gen_as.cpp at this moment.

[LucasSwitz]

@krojew Agreed. Looked over the merge into the new TS generator and it looks like combining these two would be dubious at best. I'll split out my additions into a standalone AS generator.

[MaxGraey]

Manual sign extension is unnecessary when you cast to u8 -> i32 via as i32

Suggested change

	return (i32(this.readUint8(offset)) << 24) >> 24;
	return this.readUint8(offset) as i32;

[MaxGraey]

Suggested change

	readUint32(offset: i32): i32 {
	return this.readInt32(offset) >>> 0;
	}
	readUint32(offset: i32): u32 {
	return this.readInt32(offset);
	}

[tonibofarull]

Any update on this? @LucasSwitz
You did an amazing job, thank you!

[ghost]