fix: return parser errors monadically

[wodeni]

Description

Related issue/PR: #158

Add ParserError type to Substance, Domain, and Style. The errors are just strings from nearley for now. Depending on the demand, I can try to access the parse table to customize our own errors.

Implementation strategy and design decisions

• Add ParserError to all three languages
• Minor cleaning of the repo

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 npm test
• I ran npm run 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

• How often do we need to surface parser errors? They are the least informative error, so should we try our best to avoid them in, say, the IDE by streaming to the parser and parse partial programs greedily instead?

[codecov-io]

Codecov Report

Merging #468 (c378197) into master (76ada1f) will decrease coverage by 1.24%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
- Coverage   25.82%   24.58%   -1.25%     
==========================================
  Files          74       74              
  Lines        5587     5601      +14     
  Branches     1125     1130       +5     
==========================================
- Hits         1443     1377      -66     
- Misses       4144     4224      +80     

Impacted Files	Coverage Δ
packages/penrose-core/src/utils/Error.ts	33.00% <25.00%> (-1.05%)	⬇️
packages/penrose-core/src/compiler/Style.ts	35.63% <47.05%> (-0.68%)	⬇️
packages/penrose-core/src/compiler/Substance.ts	89.95% <91.30%> (+0.28%)	⬆️
packages/penrose-core/src/compiler/Domain.ts	91.54% <100.00%> (+0.30%)	⬆️
packages/penrose-core/src/ui/styles.ts	0.00% <0.00%> (-100.00%)	⬇️
packages/penrose-core/src/packets.tsx	0.00% <0.00%> (-64.71%)	⬇️
packages/penrose-core/src/utils/Log.ts	0.00% <0.00%> (-29.42%)	⬇️
packages/penrose-core/src/API.tsx	0.00% <0.00%> (-19.24%)	⬇️
packages/penrose-core/src/ui/Embed.tsx	0.00% <0.00%> (-10.00%)	⬇️
packages/penrose-core/src/engine/Optimizer.ts	0.00% <0.00%> (-8.03%)	⬇️
... and 3 more

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