feat: svelte processor arg & improved test coverage

[tivac]

Description

The svelte preprocessor can now take a processor arg. Also the test coverage around file invalidation for <style> is improved!

Motivation and Context

I like good tests & 100% coverage makes me feel warm and fuzzy even if it's mostly worthless.

How Has This Been Tested?

I wrote tests for it! They even pass!

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

Merging #558 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   98.94%   99.04%   +0.09%     
==========================================
  Files          46       46              
  Lines        1045     1045              
  Branches      162      163       +1     
==========================================
+ Hits         1034     1035       +1     
+ Misses         11       10       -1

Impacted Files	Coverage Δ
packages/svelte/svelte.js	100% <100%> (+1.31%)	⬆️

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