Refactor and consolidate Angular Code Generator

[hypery2k]

• Code Refactorings
• Add several changes (see [typescript-angular] Packaging with ngPackagr has been lost between 2.0 and 3.0 #120, [typescript-angular] rxjs6 & providedInRoot support #236, [typescript-angular] Change RxJS imports, if using Angular >= 6 #277)
• Fixes [typescript-angular] Angular 7: An expression of type 'void' cannot be tested for truthiness #234, typescript-angular code generation has no body in POST request #347 and [typescript-angular] Generation on Windows creates paths with wrong file separator #335
• Adding some fixes from fork
• Should also solve [TypeScript] Package structure error swagger-codegen#9822

[webron]

Thanks for the submission, @hypery2k. In case you're not familiar with the option - https://github.blog/2019-02-14-introducing-draft-pull-requests.

[hypery2k]

oh thanks, didn't notice that possibility on GitHub ;)
Thanks for the hint

[hypery2k]

From my tests so far the generation with Angular 6 and 8 is working without issues.

I would like to see this PR merged and will add further functionality in another PR, ok for you @webron ?

[webron]

I leave the PR reviews to @HugoMario and @frantuma - they are way more familiar with the code here than me ;)

[HugoMario]

LGTM, thanks a lot @hypery2k!!! for this PR

[victorsempere]

Hello,
Maybe it's missing a change in the api.service.mustache? I'm getting compilation errors and I have fixed it changing the line 279:
let formParams: { append(param: string, value: any): void; };
Into:
let formParams: { append(param: string, value: any): any; };

I'm using swagger-codegen-maven-plugin 3.0.27