feat: Make `mappings` and `getMapping` public

[NikolaRHristov]

Allows for the overriding of mappings, so that they can be overriden by child themes that extend DefaultTheme.

This will resolve current issues such as https://github.com/TypeStrong/typedoc/issues/2111 with the creation of a simple theme like in https://github.com/NikolaRHristov/TypeScriptESBuild/blob/main/Source/Class/Theme.ts

[NikolaRHristov]

I just noticed I can override buildUrls, nevermind. 😁 Still useful though. Will leave it open if you want to merge.

[Gerrit0]

I'm not sure I want to merge this -- I've never liked how the URL building was implemented, and making this part of the public API seems like a step in the wrong direction...

I've been experimenting with alternative generation schemes on the beta branch, but am not fully happy with it yet

[NikolaRHristov]

Should I close the PR?

[Gerrit0]

Feel free to leave it open for now, if I vent get the rework into a state I'm happy with, I'll reconsider merging :)