fix: replace multiple missing values

[tivac]

Description

Make sure that multiple {css.key} values will be replaced if they don't exist in referenced CSS.

Motivation and Context

Fixes #575

How Has This Been Tested?

Updated specimens & snapshots, saw it fail to replace multiple values. Updated svelte.js and now they're correctly replaced.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0fc5403). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #576   +/-   ##
=========================================
  Coverage          ?   98.98%           
=========================================
  Files             ?       47           
  Lines             ?     1178           
  Branches          ?      183           
=========================================
  Hits              ?     1166           
  Misses            ?       12           
  Partials          ?        0

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

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