Add support for nested gradle projects

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Refs #86

This adds support for detecting gradle projects in nested subdirectories. I might make this configurable...

Screenshots

Here is the latest changes with nested gradle projects grouped by relative folder name. This is done to prevent conflict with sub-projects which are displayed at the workspace root.

Example of deeply nested projects:

Here is the flutter project, grouped by "android" directory name:

Here is a gradle project without nested root project directories but with subprojects:

Here is the final UI showing how the tasks are labelled in the command pallet:

TODO

• Add some basic tests

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TODO: fix the duplicated gradle task descriptions in the command pallet:

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I want to do explore the possibility of not making the task name noisy, but instead utilizing task descriptions. That way the explorer won't be noisy with redundant information, and it will be clear which task belongs to which project in the command pallet.

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I can't add task details, this seems to be an internal thing (the npm extension sets task.detail but it's of type vscode.Task2 which is not exposed).

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LGTM