feat: use XDG directories on Linux

[flexagoon]

This removes the need for the ~/.dart_tool directory in user's HOME. It moves the config file to $XDG_CONFIG_HOME/dart_tool, and all other data to $XDG_DATA_HOME/dart_tool. If the old directory exists but the new one doesn't, the old location is used as a fallback. Otherwise, the new directories are created.

See dart-lang/sdk#41560 (this doesn't fully solve the issue, but it does get rid of one of the offending directories)

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[4e554c4c]

thanks for this PR! I was thinking that we could add some utilities for this to the cli_util package and then use them across any dart programs. What do you think? See dart-archive/cli_util#94

[pauliesnug]

can you make sure this also applies to macos?