Test/browser env

[cjpillsbury]

This PR updates upchunk tests to run in a browser. To stay consistent with other projects (e.g. Open Elements, Media Chrome we've migrated to using web-test-runner. This also required some updates to tests and test dependencies that were either explicitly designed for node (nock vs. xhr-mock as a replacement) and dependencies that implicitly assumed node without an appropriate build step (e.g. xhr, which only has a published package using cjs modules).

A few callouts:

• The primary motivation for this PR is based on the ongoing effort to migrate upchunk to use ReadableStreams for memory optimization purposes, since this is unavailable (with Files) for node.
• There is one test that is currently commented out, due to some apparent assumptions in how progress events can be triggered when using xhr-mock (mentioned in code comment for further details).
• To make one test permutation easier, a read only getter for paused was added to upchunk. This feels like a low risk refactor that is also plausibly useful for non-testing cases.
• The rebuild of the ESM-equivalent modules (xhr & xhr-mock) for compatibility in the new test env currently happens before each run of test. Since these dependencies should never change, they plausibly only need to be built once after a (local/dev) install. Since the build of these modules is very fast, solving this felt like a lower priority.

[gkatsev]

An alternative to building xhr/xhr-mock each time is to check in vendored files for the tests and then have a script we can use to update them. xhr-mock was last updated in 2019 and xhr was last updated in 2020, so, incredibly unlikely they'll change.