This PR adds a custom setting projectile-auto-discover that controls whether to automatically run projectile-discover-projects-in-search-path when Projectile mode is activated.

The main purpose is to enable faster Emacs startup when starting projectile-mode automatically from the init file. On my system (~2015 i5, Emacs 27, Linux, with 140 project directories) turning off auto-discovery improves startup time by around 1.7 seconds.

The default setting for projectile-auto-discover leaves current behavior unchanged.

One already has to manually run one of the projectile-discover-* commands to update project bookmarks during an Emacs session, so it seems reasonable to allow the user to choose to always use manual updating rather than forcing an update every time they start Emacs or turn on the mode. Since project bookmarks are cached and loaded from disk, it's usually not necessary to update them anyway.

The following tests are failing on master on my machine so I don't think the failures are related to my changes. All other tests are passing.

• "projectile-grep multi-root grep grep multi-root projects"
• "projectile-compilation-dir should not fail on bad compilation dir config"

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