fix: Correct replacements of css.<key> values

[tivac]

Previously was using a raw . which would match ANY CHARACTER.

That caused all sorts of issues, with #432 being one example. Now it only replaces a literal . which is much more in-line with the original intent.

Fixes #432

[codecov]

Codecov Report

Merging #436 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   99.08%   99.08%           
=======================================
  Files          31       31           
  Lines         765      765           
  Branches      114      114           
=======================================
  Hits          758      758           
  Misses          7        7

Impacted Files	Coverage Δ
packages/svelte/src/markup.js	100% <ø> (ø)	⬆️

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