[RFC] Correctly handle multiline diagnostics in diagnostic_interface

[bstaletic]

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Why this change is necessary and useful

While there doesn't seem to be a bug here (#3993 not withstanding), there might be some improvement possible.

Previously, when we would attempt props_to_remove.remove( multiline_prop ), it was wrong 100% of the time. This PR attempts to fix that.

The thing to keep in mind is that, on vim, prop_list() never returns multiline properties. Instead, it splits multiline properties into multiple single line properties. To figure out if should skip adding a multiline property, we also need to split it by lines.

Open questions:

1. Is this a net negative in the average case, when it comes to performance?
2. Since we need to split props into single line ones, should we just always deal with single line props?
3. Does this mess anything up regarding neovim?

As for the tests... no idea how to test this thing, as it shouldn't change any directly observable behaviour.

[Please explain in detail why the changes in this PR are needed.]

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This change is 

[codecov[bot]]

Codecov Report

Merging #3997 (0b23dc9) into master (3c5a063) will decrease coverage by 0.38%. The diff coverage is 45.00%.

@@            Coverage Diff             @@
##           master    #3997      +/-   ##
==========================================
- Coverage   91.29%   90.91%   -0.39%     
==========================================
  Files          27       27              
  Lines        3712     3730      +18     
==========================================
+ Hits         3389     3391       +2     
- Misses        323      339      +16     

[puremourning]

As you said, this is quite more complexity and maybe bad for perf. So far as I know we're not seeing a lot of complaints about this. We can revisit if we do.