Support other label formats

[mtolly]

Currently just label:: is supported; label: and .label should be added.

[mtolly]

From https://github.com/bentley/rgbds/blob/master/doc/asm/labels.htm

• .label is local to a scope. A scope starts at a global label and ends at the next global label
• label: is global, visible all over the file
• label:: is global but also does EXPORT label.

On asm->mid we should support all 3 kinds and not care about exportedness (but of course match repeated occurrences of the same local label correctly). On mid->asm we should use exported labels for the channel start points, and then either use locals or non-exported globals (I'm leaning towards non-exported globals) for subroutines.