big old refactor

[Rich-Harris]

See #979. This doesn't look like much yet, but it hopefully gives a sense of my intentions. I have a good feeling about it.

A benefit that didn't occur to me before, but which is becoming obvious — this way of doing things is much more TypeScript-friendly.

[codecov-io]

Codecov Report

Merging #992 into master will increase coverage by 0.12%.
The diff coverage is 94.05%.

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
+ Coverage   91.86%   91.99%   +0.12%     
==========================================
  Files         112      115       +3     
  Lines        4169     4271     +102     
  Branches     1363     1359       -4     
==========================================
+ Hits         3830     3929      +99     
- Misses        144      148       +4     
+ Partials      195      194       -1

Impacted Files	Coverage Δ
.../generators/server-side-rendering/visitors/Slot.ts	100% <ø> (ø)	⬆️
...generators/server-side-rendering/visitors/index.ts	100% <ø> (ø)	⬆️
src/generators/server-side-rendering/index.ts	98.07% <ø> (-0.04%)	⬇️
src/generators/wrapModule.ts	91.02% <ø> (ø)
src/generators/Generator.ts	94.38% <100%> (+0.31%)	⬆️
src/generators/nodes/RawMustacheTag.ts	100% <100%> (ø)
src/generators/nodes/Transition.ts	100% <100%> (ø)
src/css/Stylesheet.ts	89.9% <100%> (+0.19%)	⬆️
src/utils/createDebuggingComment.ts	100% <100%> (ø)
src/generators/nodes/MustacheTag.ts	100% <100%> (ø)
... and 49 more

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[Rich-Harris]

Think this is probably in good enough shape to merge. Still more that could be done but the foundations have been laid.