fix: improve replacement to work with extending from node_modules

[dominikg]

fixes #188

The issue was rebaseRelative prepending segments in front of ${configDir} and the replacer at the end did not work with that.

Fixed by skipping rebaseRelative if value starts with ${configDir}

[Zamiell]

i would recommend depending on complete-tsconfig instead of isaacscript-tsconfig, since I intend to change isaacscript-tsconfig in the future, and then dependobot would cause your test to fail i think.