Fix NABC inter-syllable overflow spacing and add nabcintersyllablemingap

[lbssousa]

Fixes #1715.

[davidweichiang]

In #1720 I'm working on moving everything related to syllablefinalskip into Lua, while this PR adds a dependency on syllablefinalskip. So merging this PR first would create some extra work needed on #1720.

On the other hand, if #1720 were merged first, then managing the space between NABC syllables would become much easier. It would just involve adding a pass to the Lua pre_linebreak filter that looks for consecutive nodes with part=7, and if the distance between them is less than 0, increase the syllablefinalskip node by that amount. Similarly for part=8.

Maybe #1720 can reach a point where it's mergable in a few days. @lbssousa, what do you think about trying this in Lua instead?

[lbssousa]

Sure, @davidweichiang . I'll try it when your work is ready. We can hold this PR for now.