Part of
https://linear.app/zed-industries/issue/Z-1386/add-equivalent-setting-to-vscodes-auto-reveal-exclude
Deals with https://github.com/zed-industries/community/issues/800

* add a `project_panel::auto_reveal_entries` config entry to allow
disabling auto reveal in project panel (auto reveal is enabled by
default)
* add a `pane::RevealInProjectPanel` action (and a pane tab context menu
entry) to manually reveal any file entry
* stop auto revealing gitignored directories at all

We can add the auto reveal exclude globs later, if needed, but let's try
to keep the config simpler and start with a more minimalist approach.

Release Notes:

- Improved project panel auto reveal mechanics: gitignored files are not
auto revealed anymore; a `auto_reveal_entries = true` config option for
`project_panel` is added; a `pane::RevealInProjectPanel` action and a
corresponding buffer tab context menu were added