Option to define URL transformation

[nstringham]

My markdown contains links with relative paths like [a source code file](/src/path/to/file.java) I want this to get transformed to [a source code file](https://github.com/somedude/somerepo/tree/main/src/path/to/file.java) this could be accomplished by adding an option in vscode settings that I can set to https://github.com/somedude/somerepo/tree/main and that will get pretended to every url that doesn't contain ://

I am willing to try to help make a PR for this if that would be helpful.