set typescript target to ES6

[Conduitry]

Two issues here - one is that the target option actually belongs inside compilerOptions in tsconfig.json, and so TypeScript was actually using its default target of ES3, not ES5. And then also there's not really a reason to target ES5 as far as I can tell anyway. This shaves 9% off the bundle size.

[Conduitry]

I've also removed the es5, es6, and dom from the lib compiler option. I'm ... not entirely clear on what these do. But Svelte builds and the tests pass locally for me without them. I feel less strongly about this second change, and would understand if you were hesitant to include it. Nevermind, I think I know vaguely what that is now, and it's not something we want to change. Removed that commit. What I'm not sure of, though, is why this reduces test coverage...

[codecov-io]

Codecov Report

Merging #1261 into master will decrease coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1261      +/-   ##
==========================================
- Coverage   91.88%   91.45%   -0.44%     
==========================================
  Files         126      119       -7     
  Lines        4548     4283     -265     
  Branches     1478     1348     -130     
==========================================
- Hits         4179     3917     -262     
+ Misses        153      152       -1     
+ Partials      216      214       -2

Impacted Files	Coverage Δ
src/utils/list.ts	50% <0%> (-16.67%)	⬇️
src/generators/nodes/Title.ts	54.05% <0%> (-6.42%)	⬇️
src/generators/nodes/MustacheTag.ts	87.5% <0%> (-4.17%)	⬇️
src/generators/server-side-rendering/index.ts	93.84% <0%> (-1.93%)	⬇️
src/css/Selector.ts	91.15% <0%> (-1.56%)	⬇️
src/generators/nodes/Text.ts	93.75% <0%> (-1.49%)	⬇️
src/css/Stylesheet.ts	90.1% <0%> (-1.26%)	⬇️
src/utils/annotateWithScopes.ts	84.78% <0%> (-1.22%)	⬇️
src/generators/nodes/Attribute.ts	84.74% <0%> (-1.05%)	⬇️
src/utils/CodeBuilder.ts	85.33% <0%> (-0.88%)	⬇️
... and 33 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bc416a5...60a7341. Read the comment docs.

[Rich-Harris]

SHUT UP CODECOV, YOU'RE NOT MY MUM