Add Breadcrumb Panel (ST4+, GitHub tags)

[sarev]

• I'm the package's author and/or maintainer.
• I have have read the docs.
• I have tagged a release with a semver version number.
• My package repo has a description and a README describing what it's for and how to use it.
• My package doesn't add context menu entries. *
• My package doesn't add key bindings. **
• Any commands are available via the command palette.
• Preferences and keybindings (if any) are listed in the menu and the command palette, and open in split view.
• [n/a] If my package is a syntax it doesn't also add a color scheme. ***
• I use .gitattributes to exclude files from the package: images, test files, sublime-project/workspace.

My package is similar to the Breadcrumbs package except it is a cleanroom implementation that outputs to a (semi-persistent) panel, similar to how the linters often work, meaning you can always have a view of your breadcrumb context for the active view while you're editing. I've taken steps to ensure it isn't too much of a performance drain and been very generous with the comments to hopefully help other plugin developers to understand how it works.