Support sentry-webpack-plugin release injection in browser and node SDKs

[tstirrat15]

Fixes #2107
Fixes getsentry/sentry-webpack-plugin#126

Motivation

See #2107 and this comment on the sentry-webpack-plugin issue.

sentry-webpack-plugin adds the following to the global scope (whether backend or frontend):

{ SENTRY_RELEASE: { id: 'your-id-string' } }

It's picked up automatically by raven-js (though, interestingly, not by raven-node).

This work adds the same functionality to the new browser and node SDKs.

Changes

• Add clarification to the CONTRIBUTING.md file that you have to run yarn build before you can run tests against any of the packages
• Add tests that capture the desired behavior
• Add references to the global SENTRY_RELEASE variable if it's available

[HazAT]

Thanks 💯

[HazAT]

Fixes #2107