[WIP] fix some TODOs

[Rich-Harris]

Lots of TODO comments in the codebase — some of them are probably fairly easy to fix

[codecov-io]

Codecov Report

Merging #1193 into master will increase coverage by <.01%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #1193      +/-   ##
==========================================
+ Coverage   91.69%   91.69%   +<.01%     
==========================================
  Files         127      126       -1     
  Lines        4564     4566       +2     
  Branches     1505     1495      -10     
==========================================
+ Hits         4185     4187       +2     
- Misses        157      158       +1     
+ Partials      222      221       -1

Impacted Files	Coverage Δ
src/generators/dom/index.ts	96.21% <ø> (ø)	⬆️
src/generators/Generator.ts	94.01% <ø> (-0.1%)	⬇️
src/generators/dom/Block.ts	96.82% <ø> (ø)	⬆️
src/validate/js/utils/usesThisOrArguments.ts	81.81% <ø> (ø)	⬆️
src/validate/html/index.ts	97.87% <100%> (+0.04%)	⬆️
src/parse/state/tag.ts	96.4% <100%> (+0.04%)	⬆️
src/generators/nodes/Slot.ts	91.04% <100%> (+0.88%)	⬆️
src/generators/nodes/MustacheTag.ts	91.66% <50%> (-8.34%)	⬇️
src/generators/nodes/RawMustacheTag.ts	97.67% <50%> (-2.33%)	⬇️
src/generators/nodes/Text.ts	95.23% <50%> (-4.77%)	⬇️
... and 5 more

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