Draft: base: implement the TODO pattern and the todo function

Description

This adds the TODO pattern and the todo function to indicate unfinished code. It is supposed to fill an ergonomic and semantic gap in the current base library.

This function is subject to discussions at issue 260 of the core libraries committee's github repository

It is clear to me that this MR still requires tests which will be added by my as soon as possible. For now this may serve as the code for the core libraries committee to evaluate when they make the decision of whether this shall become part of base.

screenshot of the generated haddock html

Done

• if your MR may break existing programs (e.g. touches base or causes the compiler to reject programs), please describe the expected breakage and add the user-facing label. This will run ghc/head.hackage> to characterise te effect of your change on Hackage.
• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.

Todo

• add a testcase to the testsuite.
• updates the users guide if applicable
• mentions new features in the release notes for the next release