chore(browser): swap URL polyfill

[lukeed]

Hey there,

I put together a 1.5kB polyfill specifically to unblock this issue: #400

We'd really like to move forward with source-map integration within the Svelte REPL environment, but not at the expense of nearly doubling its javascript! With whatwg-url included – which is rarely needed (especially on a developer machine) – source-map weighs 30% more than CodeMirror.

This polyfill, like whatwg-url, polyfills the URL implementation from Node.js instead of the browser's. It also passes the linked Chromium & Mozilla issues.

[coveralls]

Pull Request Test Coverage Report for Build 578

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.713%

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Totals
Change from base Build 576:	0.0%
Covered Lines:	2481
Relevant Lines:	2733

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💛 - Coveralls

[lukeed]

Hey :) I know we're all busy with the holidays, but just wanted to check in. If this has no chance of landing, that's totally fine, but please let me know so that our side can start formulating backup plans in order to move forward.

Thanks! ☃️