feat: add UserActionInstrumentation

[joaquin-diaz]

Goal

Add the first iteration of the user action instrumentation only covering the mousedown event based on the current semantic conventions PR

Extras

Vitest

Simple vitest setup for running unit tests using jsdom

Initial monorepo setup

Preparing for a future turbopack integration, I set up some packages for the upcoming monorepo setup.

• instrumentation-user-action: package where the new instrumentation lives
• test-utils: a private shared package where test utilities can live
• web-utils: a private shared package where cross instrumentation web utils can live

Each package has these set of scripts:

• test: vitest run
• test:watch: vitest
• test:coverage: vitest --coverage

Once we set up a monorepo tool (possible turbopack) we can easily run these commands from root

TODO: make sure package can build

What's not included

• CI/CD: still not hooked up, will wait until we have turbopack
• Release/publishing: not yet; we need to review the release strategy with the sig

[martinkuba]

There is an existing web-common package. It might be confusing to have two similar packages published in npm. I suggest that we either move web-common to this repo, or add these utils there for now.