Handling of .gitignore in subdirectories

[blaugold]

Nx only seems to take the .gitignore at the workspace root into account. add-nx-dart-to-monorepo does this too, since it's based on add-nx-to-monorepo.

• add-nx-dart-to-monorepo should refuse to proceeded when .gitignores in subdirectories exist.
• The documentation should make it explicit that monorepos are expected to have only one .gitignore at the workspace root.
• add-package generator should handle removing .gitignores in packages.

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Update: It would be a lot less complicated to just have .gitignore files work as expected. .gitignores in Flutter packages can be quite elaborate and are generated by the flutter tool when the packages is created or when platforms are added. They also contain ignore rules that are relative to the .gitignore. Automatically merging those into a single .gitignore reliably, is a non-trivial task.

Update: PR to add support for nested ignore files to Nx nrwl/nx#10455