Rework map_selection.sample.lua

[maritaria]

• Include a list of available maps
• Rewrite instructions

[maritaria]

Note: I've not yet gone through all available options and verify that each works. I basically made a list of all generators in paths map_gen/maps/*.lua and map_gen/maps/*/preset/*.lua. Some areas are not covered like the april-fools one.

I made this PR because I think this format is a bit more noob-friendly.
Perhaps having the list of options in a giant comment for them to copy/paste would be easier for them to use than to uncomment lines, but I'll leave that up to you.

[maritaria]

/rej

[maritaria]

(closed, will reopen with similar idea).

Turns out there is something wrong with having code after top-level return.

EDIT: Lua complains about statements after top-level return. So this approach would only work if user only ever uncomments exactly one return require line. I'll rework to more closely resemble old setup but with full list of all options in the comment area.