🚀 Add delta doctor command

[dandavison]

There should be a command named delta doctor or delta --doctor which checks various aspects of the user's environment and reports on anything that seems like it will cause problems with delta usage.

For example, this could check for:

• less version (see the many issues from Windows users with broken installed version of less)
• git environment variables that are not supported by delta (#1113)
• GIT_PAGER env var that doesn't contain "delta"
• *PAGER env var using a pager other than less
• using tmux/screen without the required obscure terminal config hacks
• 24bit color support and box-drawing unicode characters support etc (at least, it could print out some content and link the user to an image showing what it should look like)
• cannot read git config
• essential delta config does not seem to be present in git config
• Check for deprecated delta keys in config (see https://github.com/dandavison/delta/issues/912)
• Check for unknown keys or apparently mis-spelled keys
• Check whether user is using latest delta version

Perhaps it should be possible to silence selected warnings via delta config.