feat: project_stash_preview

[tanvirtin]

Closes: https://github.com/tanvirtin/vgit.nvim/issues/248

• Executing the command :VGit project_stash_preview will open a VGit preview of all stashed entries in your project.
• Users can press to select the stashes
• Pressing enter will show all changes associated with the selected stashes in project_commits_preview.
• If nothing is selected, then the entry on the current line will be sent to project_commits_preview