[rush] Support --changed-projects-only in watch mode

[dmichon-msft]

Summary

Defines a behavior for --changed-projects-only in Rush watch-mode phased commands.

Details

• For the initial build, does nothing, since there is no concept of "project that doesn't need to be built"
• For watch-mode rebuilds, only operations for projects that have changed files, or downstream operations that specify ignoreChangedProjectsOnlyFlag in rush-project.json will be executed.

You can toggle the behavior on or off between iterations by pressing c on the command line.

How it was tested

Local run of rush start --to heft-webpack5-everything-test against a different worktree.

Impacted documentation

Documentation for the command line parameter. The toggle is documented in the CLI UX.