Support Glyphs-style "tuples"

[simoncozens]

Glyphs 3 plist files have "tuples" which appear on a single line even when other lines are separated/indented. The easiest way to support this would be to remove the if indent is not None... blocks from write_array_from_tuple. However, this may have Unintended Consequences.

[anthrotype]

ugh. I really wish we can avoid adding quirks to this project, I'd like to keep it general purpose and not tied to Glyphs.app, even if it's the main target. Can we think about ways to inject some custom callbacks to format these maybe?

[anthrotype]

on second thought, I guess it'd be fine if we add a new writer option, e.g. single_line_tuples=False, that optionally enable this. I see we already have two duplicate implementations for serializing list and tuple objects. Could you send a PR?