fix: Get rid of istanbul-instrumenter-loader

[mjameswh]

What changed

In @temporalio/nyc-test-coverage:

• Remove dependency on istanbul-instrumenter-loader
• Replaced by a private webpack module loader
• Add integration tests
• Modify rules to make sure that all user source files are included in coveerage

Why

• 'istanbul-instrumenter-loader' was causing inclusion of vulnerable package ' [email protected]'
• project 'istanbul-instrumenter-loader' is no longer maintained (it has actually been archived)
• fix nyc-test-coverage-1.5.2.tgz: 2 vulnerabilities (highest severity is: 9.8) - autoclosed #994, fix [Bug] nyc-test-coverage uses unmaintained istanbul-instrumenter-loader #911, fix istanbul-instrumenter-loader-3.0.1.tgz: 1 vulnerabilities (highest severity is: 5.6) #872

[vkarpov15]

LGTM just from reading. I've been having some trouble running this locally, but I get the same issues when running without this change. Will keep trying.

[vkarpov15]

Managed to get this working locally successfully. Turns out the issue that I was experiencing only happened when I tried to install a patched version of nyc-test-coverage with npm pack.