feat: support emitting abstract classes for implementation types

[mnahkies]

introducing a new cli flag --ts-server-implementation-method <value> that controls whether
the "implementation" contract for each router is emitted as a:

• type (default, previous behavior)
• interface
• abstract class

for the abstract class case I've played around with using this with the diod DI framework over here https://github.com/mnahkies/node-scim/pull/1/files?diff=unified&w=1 and it seems to work fine, albeit its annoying having to alias the imported Implementation and createRouter symbols (this has been bugging me for a while tbh, but out of scope for this change)

[ADRFranklin]

So far this looks really good, so no complaints from me and I do remember also mentioning the problem with Implementation as the name of the interface (#111 (comment)) as I also found it to be cumbersome renaming the implementation when registering the interface with the implementation. So would love to see a fix for that at some point.

Thanks for writing up a draft so quickly, did not expect something so fast :D