Selection snapping for associative terms [draft]

[nskh]

Enable Hazel to snap to proper subterms of an expression (i.e. highlight all of 1 + 2 when user selects over just, say, + 2 or 1 +). Handle left-associative binary operators for arithmetic by traversing the AST (described in a comment).

Still needs some work. Action items here:

• behavior is a little odd over let = in, likely due to how that form is represented in the zipper
• parenthesize and then run statics in Stepper contexts (which add some parens that aren't findable when we look up from statics.info_map)
• pull this logic out of Deco and place in StepperEditor only
• extend or refactor to generic exp-based selection

Later:

• take hidden steps to reparenthesize parts of the AST that are selected before taking Stepper actions, so we can act on proper sub-terms instead of gating that behind only the outer-most left-associative term

[cyrus-]

@nskh we just merged a PR where @disconcision rewrote some of the selection code, feel free to ping him

[nskh]

@cyrus- #1906? I'll take a look.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.95%. Comparing base (d601b04) to head (846e315).

Files with missing lines	Patch %	Lines
src/web/app/editors/decoration/Highlight.re	0.00%	59 Missing ⚠️
src/haz3lcore/zipper/Zipper.re	0.00%	19 Missing ⚠️
src/language/term/Operators.re	0.00%	10 Missing ⚠️
src/web/app/editors/code/CodeEditable.re	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1890      +/-   ##
==========================================
- Coverage   51.28%   50.95%   -0.34%     
==========================================
  Files         205      205              
  Lines       21754    21841      +87     
==========================================
- Hits        11157    11128      -29     
- Misses      10597    10713     +116     

Files with missing lines	Coverage Δ
src/web/app/editors/code/CodeEditable.re	0.00% <0.00%> (ø)
src/language/term/Operators.re	77.46% <0.00%> (-1.89%)	⬇️
src/haz3lcore/zipper/Zipper.re	56.72% <0.00%> (-4.93%)	⬇️
src/web/app/editors/decoration/Highlight.re	0.00% <0.00%> (ø)

... and 14 files with indirect coverage changes

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