feat: add resolution of `$ref`s in subdirectories

[aeworxet]

This PR:

• Adds resolution of $refs in subdirectories.

• Refactors the application's code, getting the codebase ready for future implementations.

Resolves https://github.com/asyncapi/bundler/issues/178

[Laupetin]

Tested it on my example and it works fine. The problem I had should be solved with that, thanks :)

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[aeworxet]

/rtm

[asyncapi-bot]

:tada: This PR is included in version 0.6.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: