feat: check existence and type of labels in Style selectors

[wodeni]

Description

Closes #774

In this PR, we introduce a new Style selector pattern: X has <math|text> label. When included in the where clause of a selector, the compiler checks if label exists and whether it matches with the specified descriptor. Examples:

• Label x $\vec{x}$ will match with where x has math label
• Label x "xxxx" will match with where x has text label
• Both of the above will match with where x has label
• NoLabel x will not match with where x has label nor where x has math/text label

Error handling:

• Only math and text are valid field descriptors. All others will result in parse errors
• label is the only field name allowed, but instead of throwing a parse error, the compiler reports SelectorFieldNotSupported error:

Cannot match on field x.blah (line 14, column 31 of Style program) because matching on fields is not fully supported. Currently, only "label" can be matched in selectors.

Implementation strategy and design decisions

• Introduce rel_field rule in the Style parser
• Add RelField type in the Style compiler
• In Substance postprocessing, attach the type of the label to entries in labelMap
• In the Style compiler, check for the existence of x and the label existence and type during selector matching
• Add tests in Style parser, Substance compiler, and Style compiler

Examples with steps to reproduce them

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass locally using yarn test
• I ran yarn docs and there were no errors when generating the HTML site
• My code follows the style guidelines of this project (e.g.: no ESLint warnings)

Open questions

There's no infrastructure for testing selector matching results, so we are not including tests for checking selector matching results.

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Codecov Report

Merging #777 (a280fce) into main (52c8005) will decrease coverage by 0.15%.
The diff coverage is 56.75%.

@@            Coverage Diff             @@
##             main     #777      +/-   ##
==========================================
- Coverage   65.56%   65.40%   -0.16%     
==========================================
  Files          53       53              
  Lines        7939     7978      +39     
  Branches     1460     1409      -51     
==========================================
+ Hits         5205     5218      +13     
- Misses       2725     2751      +26     
  Partials        9        9              

Impacted Files	Coverage Δ
packages/core/src/compiler/Style.ts	86.24% <53.12%> (-1.52%)	⬇️
packages/core/src/utils/Error.ts	34.64% <60.00%> (+0.56%)	⬆️
packages/core/src/compiler/Substance.ts	91.20% <100.00%> (+0.02%)	⬆️

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[samestep]

Ah so you changed if/else if/else to switch to avoid having the else case which throws? Nice 👌

[wodeni]

Ah so you changed if/else if/else to switch to avoid having the else case which throws? Nice 👌

Yep! Still puzzling why there's a distinction in the first place, but yeah it's better to have static errors when types change, instead of these weird exceptions.

I'm just change them as I go. There are a lot more in the repo. Maybe do a full pass later.

[samestep]

Looks good to me! 👍