fix!: Improve error handling when environment fetch fails. Minor type safety breaking changes

[rolodato]

This PR fixes #177, which is tested here: https://github.com/Flagsmith/flagsmith-nodejs-client/pull/178/files#diff-d51f632ca0e8d754d607e28740780bb82e314eb5b22dbba97edd8f2a7c7cb32cR344.

Breaking changes introduced in this PR:

• this.environment is typed as not null, but this is not true. This should never have been exposed, and now we have a getEnvironment method that exposes a shared promise for the environment document
• The environment argument for onEnvironmentChange handlers can now be undefined, which happens if there was an error. If we failed to update the environment, it doesn't make sense to send the previous environment as part of this event.
• Flagsmith constructor now throws an error if local evaluation is enabled but a server-side key was not used. Previously it would log an error and continue

This PR also makes it so that the test suite does not cause actual API calls to Flagsmith. All HTTP calls in tests are now mocked.

Other misc fixes:

• Setting environmentRefreshInterval: 0 prevents polling from starting
• getIdentityFlags now actually uses a default flag handler if provided, or fails if it needs to be called but is missing. Previously this would fail and not use the default handler
• Remove use of console.log and console.error, always use loggers

New features:

• requestRetryDelayMilliseconds option to make retry delay configurable, previously hardcoded at 1 second. This, with the default 3x retries, made some tests take at least 3 seconds to finish. Now, we do 0 retries and 0 delay in tests by default.