Supports vscode-webview-resource: source maps

[eamodio]

Adds source-map support for vscode-webview-resource:// urls from vscode webviews

[connor4312]

From slack discussion -- webview resource uris take the form

vscode-webview-resource://ac8c06dc-010a-4ea5-bbda-43d6f25dd21c/file///c%3A/Users/Eric/code/eamodio/vscode-gitlens/dist/webviews/rebase.js.map

That uri has file/// instead of file:///. I think this might work by going through got which can normalize it, but this is more incidental than intentional -- I'd prefer we fix up the protocol so this doesn't break in the future.

This is the code in VS Code which handles decoding: https://github.com/microsoft/vscode/blob/ee0aff2460957b825fb4ce2edc3a6d21c90c2c04/src/vs/platform/webview/common/resourceLoader.ts#L104

you can use the standard URL instead of vscode URIs in this case. I would keep the cheap candidate.startsWith('vscode-webview-resource://') check (rather than checking new URL(url).protocol, since passing data URIs in new URL() has caused performance issues.