implement syntax changes

[Rich-Harris]

This duplicates all the parser tests with a -v2 suffix, and enables opting in to the new syntax with the parser: 'v2' compiler option. When it comes time for v2, we can just remove all the old stuff.

[codecov-io]

Codecov Report

Merging #1330 into master will decrease coverage by <.01%.
The diff coverage is 90.1%.

@@            Coverage Diff             @@
##           master    #1330      +/-   ##
==========================================
- Coverage   91.72%   91.72%   -0.01%     
==========================================
  Files         125      125              
  Lines        4472     4505      +33     
  Branches     1401     1430      +29     
==========================================
+ Hits         4102     4132      +30     
- Misses        146      149       +3     
  Partials      224      224

Impacted Files	Coverage Δ
src/validate/html/index.ts	97.87% <ø> (ø)	⬆️
test/parser/index.js	100% <ø> (ø)	⬆️
...rators/server-side-rendering/visitors/Component.ts	96.77% <100%> (ø)	⬆️
src/parse/state/text.ts	100% <100%> (ø)	⬆️
src/parse/read/expression.ts	100% <100%> (ø)	⬆️
src/generators/nodes/Component.ts	96.9% <100%> (ø)	⬆️
src/parse/state/fragment.ts	100% <100%> (ø)	⬆️
src/generators/Generator.ts	96.68% <100%> (ø)	⬆️
src/utils/createDebuggingComment.ts	100% <100%> (ø)	⬆️
src/generators/nodes/EachBlock.ts	97.63% <100%> (ø)	⬆️
... and 8 more

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[Rich-Harris]

Made a tiny bit more progress. I've also built a tool for automatically updating your components, which we can use to thoroughly test this all out prior to release:

https://github.com/sveltejs/svelte-upgrade

[Rich-Harris]

I think everything is accounted for now. This effectively pulls the trigger on v2 — it will be costly to make further changes while both syntaxes are supported, so in a bit I'll put together a TODO list of what else needs to happen for v2. There's a GitHub project for it, but it's currently incomplete.