add skipIntroByDefault compiler option

[Rich-Harris]

Fixes #547. This is the first step towards overhauling the transition system to enable more complex effects. It adds a new skipIntroByDefault compiler option which is similar to cascade: false and store: true in that it opts you in to behaviour that I think should be the default in v3.

The intro: true initialisation option is then needed to opt you back in to intro transitions on first render. Transitions inside blocks whose values change after the initial render are unaffected.

[codecov-io]

Codecov Report

Merging #1448 into master will decrease coverage by 0.1%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
- Coverage   91.09%   90.98%   -0.11%     
==========================================
  Files         122      122              
  Lines        4492     4495       +3     
  Branches     1406     1410       +4     
==========================================
- Hits         4092     4090       -2     
  Misses        158      158              
- Partials      242      247       +5

Impacted Files	Coverage Δ
src/compile/nodes/Component.ts	92.82% <0%> (-1.67%)	⬇️
src/compile/dom/index.ts	95.27% <50%> (-0.73%)	⬇️

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