[rush:subspace] Add BaseFlag and --ignore-scripts

[william2958]

Summary

This MR adds many useful changes introduced in the split-workspace PR (https://github.com/microsoft/rushstack/pull/3481) such as the creation of the BaseFlag class as well as the addition of the --ignore-scripts parameter, which allows for installation without having to run the post-install scripts.

Note that the --ignore-scripts feature is also introduced here: https://github.com/microsoft/rushstack/pull/3537 but the PR is a bit outdated but it was also referenced for this PR.

Both of these features will be very helpful for the installation implementation of subspaces, so I thought it would be easier to introduce them first.

Details

How it was tested

Impacted documentation