Add `dart create` flag to add new package to parent workspace

[parlough]

It would be nice to have a flag for dart create that if specified (perhaps --workspace), traverses ancestor directories, and if it finds a package with an existing workspace key, adds the new package to that workspace. It would also add resolution: workspace to the new package. If no parent workspace is found, the command should fail with an error rather than trying to create the package.

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Related: #61237

\cc @sigurdm

[sigurdm]

We could also have dart pub workspace add that adds a directory to its surrounding workspace?