Allow configuring sections

[ISSOtm]

Currently, each script is emitted as an individual ROMX section.

• One may want to emit scripts to ROM0, for example (and, if RGBDS later allows arbitrary section kinds via hardware description files, others);
• One may want all scripts to lie within a single section.

I think this could be solved via a CLI switch; in particular, if the section type given is empty, no section directives would be emitted.

[evie-calico]

In my original drafts of the language (and maybe the c++ version?), this was determined by the env; i must have just forgotten it.

That being said, i don't think a command line flag is the right option. I think the best way to do it would be to define a default per env, with overrides for individual scripts. The only issue is that i don't have any ideas for the latter's syntax.

[ISSOtm]

Perhaps mimicking the pool = syntax? I'd implement it by making the LHS into a regular ident instead of piling on keywords, since those'd quickly clog up the available names for variables and functions.