Simplify Suspense code and add finished callback to transition.start

[lukechu10]

No description provided.

[codecov]

Codecov Report

Merging #414 (44d8aa0) into master (e21b576) will decrease coverage by 0.14%.
The diff coverage is 92.30%.

❗ Current head 44d8aa0 differs from pull request most recent head cbfd847. Consider uploading reports for the commit cbfd847 to get more accurate results

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
- Coverage   64.16%   64.01%   -0.15%     
==========================================
  Files          49       49              
  Lines        7886     7862      -24     
==========================================
- Hits         5060     5033      -27     
- Misses       2826     2829       +3     

Impacted Files	Coverage Δ
packages/sycamore/src/suspense.rs	86.66% <92.30%> (-3.64%)	⬇️
packages/sycamore-reactive/src/effect.rs	99.31% <0.00%> (-0.03%)	⬇️
packages/sycamore-reactive/src/lib.rs	95.45% <0.00%> (ø)
packages/sycamore-reactive/src/arena.rs	100.00% <0.00%> (ø)

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