Add browser extension log field

[ka3de]

Description of changes

Adds a new source:browser log field which allows to distinguish log messages generated by xk6-browser extension from messages generated by k6 core. As mentioned in #934 (comment), the source label is currently used across k6 codebase.

An example of different log messages after the change applied in this PR including:

• One debug message from k6 browser implementation
• One log message from browser context execution
• One console.log message from browser context execution
• One console.log message from k6 test script execution.

DEBU[0000] wsURL:"ws://127.0.0.1:34591/devtools/browser/dc9e1e8b-058e-47c4-97df-3b4dfa2d81a7"  category="Browser:connect" elapsed="0 ms" iteration_id=dda9fe94f08fa83b source=browser
...
ERRO[0001] Failed to load resource: the server responded with a status of 403 ()  browser_source=network line_number=0 source=browser stacktrace="<nil>" url="https://grafana.com/canspam"
INFO[0001] "This is a console.log message from browser context execution"  browser_source=console-api source=browser
INFO[0001] This is a console.log message from k6 test script context execution  source=console

Closes #934

[ankur22]

LGTM 🚀

[ka3de]

A note on a090ffc:
I considered initially to refactor our Logger implementation in order to support WithTime and WithField method, but this was deriving in a decent refactor, which I think is better to tackle in a different PR if we want. Also, currently there are only two methods (onLogEntryAdded and onConsoleAPICalled) that build a new logrus Logger independent from our higher level abstraction, both related with browser console output, so we might not want to currently support this anyway.

[ankur22]

LGTM 🚢 it!