refactor: pull out custom rules code

[teodora-sandu]

What does this PR do?

This PR extracts the custom and internal Snyk rules code from the main test function into its own file. For increased readability and in preparation of adding extended Terraform language support, which will make this function even bigger. This is needed for #2752 (which is not ready for review yet and pending snyk/snyk-iac-parsers#7).

How should this be manually tested?

To review, have a look at the new initRules function and notice how it matches up with the code extracted from the test function.

To test it:

1. npm run build
2. Create a custom rules bundle using https://github.com/snyk/snyk-iac-rules: snyk-iac-rules build ./fixtures/custom-rules -o bundle.tar.gz
3. Run the CLI on a file that would trigger the rule (just use the fixtures in that repo): snyk-dev iac test ./fixtures/custom-rules/rules/CUSTOM-1/fixtures

The acceptance tests would verify this plus the remote bundles, so if the tests pass then this is good to go.

[github-actions]

	Warnings
⚠️	You've modified files in src/ directory, but haven't updated anything in test folder. Is there something that could be tested?

	Messages
📖	This PR will not trigger a new version. It doesn't include any commit message with feat or fix.

Generated by 🚫 dangerJS against e791e19

[ofekatr]

Excellent initiative, the main flow feels much cleaner after this refactor.
Managed to successfully run it locally 🚀
Left a small refactor suggestion open for discussion.

[ofekatr]

This is not a request, but rather an idea open for discussion.

I've given a bit of consideration to the return type of this function.
What we have here seems to be a tri-state value for provided custom rules.
Custom rules are either:

1. Not provided.
2. Provided locally.
3. Provided remotely.

Instead of returning an object with 2 boolean flags, one for flagging rules provided locally, and one for flagging provided remotely, I'd like to propose returning a union type value here:

type RulesOrigin = 'local' | 'remote' | 'none';

Which can later be utilized in the addIacAnalytics function, where these values are needed.

[ofekatr]

Alternatively, the return value can be optional for the case where rules are not provided, and so the union values could be shortened to be:

type RulesOrigin = 'local' | 'remote';

[teodora-sandu]

That was a really good suggestion. Makes the code much easier to ready 🤩