fix: ignore URLs in <link href>

[tivac]

Description

• Adds a warning if there are multiple <link> elements in a file, processes only the first
• Ignores elements when the href attribute is a URL
• Early-out if all the <link> elements are to URLs
• Removes trailing newlines when stripping out replaced <link>, which looks nicer

Motivation and Context

Wanted to be able to mix local <link> that should be transformed with external CSS that shouldn't be.

How Has This Been Tested?

Tests!

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 #553 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
+ Coverage   98.92%   98.94%   +0.01%     
==========================================
  Files          46       46              
  Lines        1026     1041      +15     
  Branches      157      160       +3     
==========================================
+ Hits         1015     1030      +15     
  Misses         11       11

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

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