Skip invalid urls

Do not add trailing / for no reason

fix(resolvers): dn't resolve external references

chore(server-url-placeholder): enVar for server objects

fix(paw-url): refactor url generation

this commit refactors PawURL class, where it utilizes URL class
instead of RegEx to detect a valid url.

Fixes for issue:
- luckymarmot#11: import is empty - almost
- luckymarmot#12: not work on openapi 3.0.3

fix(paw-url): refactor url generation

this commit refactors PawURL class, where it utilizes URL class
instead of RegEx to detect a valid url.

Fixes for issue:
- luckymarmot#11: import is empty - almost
- luckymarmot#12: not work on openapi 3.0.3

chore(enable-circular-reference)

- updated: openapi v3 petstore example
- updated: enable circular reference

chore(improvements): import server object

* Suggest a shorter name. Remove redundant “Importer” name.
* Fix import from URL
* Import server object variables and use them as env variables

chore(improve-importer): init

Co-authored-by: Micha Mazaheri <micha@mazaheri.me>

- updated: utilize @apidevtools/swagger-parser to validate and parse OpenAPI documents
- updated: utilize [openapi-types](https://github.com/kogosoftwarellc/open-api/tree/master/packages/openapi-types#readme) for typings
- chore: upgraded webpack, babel setup and jest setup
- chore: updated extension config references
- chore: simplify code coherence, reduce code complexity

**TODO**
- [ ] `utils/paw-url.ts` improve code coherence, utilize `URL` api
- [ ] `utils/json-schema-parser.ts` - improve code, ensure compliance with [openapi/json-schema](https://tools.ietf.org/html/draft-wright-json-schema-00#section-4.2) specs
- [ ] `utils/console.ts` - ensure to cover cyclic objects+references

Co-authored-by: Micha Mazaheri <micha@mazaheri.me>

 init