Make base of relative pattern a URI instead of string

[bpasero]

Today IRelativePattern.base is a string and we cannot change that in our APIs. However, when this value goes over the wire from the extension host to the window, we should transport it as URI so that we get the right origin.

The change in c864113 was a bandaid for #99938 and can further be improved.

//cc @aeschli

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