Simplify spoiler regex

[spenserblack]

This fixes an issue where trailing characters after the spoiler would severely effect performance.

Fixes #123

[vercel[bot]]

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Name	Status	Preview	Comments	Updated
steamdown	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Mar 18, 2023 at 7:18PM (UTC)

[codecov[bot]]

Codecov Report

Merging #125 (e18d195) into main (621c3bf) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #125   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files           3        3           
  Lines         244      244           
  Branches       47       47           
=======================================
  Hits          243      243           
  Partials        1        1           

Impacted Files	Coverage Δ
packages/steamdown/src/extensions.ts	99.44% <100.00%> (ø)

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