feat(MarkerStack): add MarkerStack package

[vwheeler63]

MarkerStack Package:

Push/pop cursor (caret) position in View with single keystroke; marks position in left gutter; saves state between sessions. This feature enables you to interrupt a task to change something ELSE in the same file, and then return to a prior position and viewport position. This interruption can be nested to any depth, thus the STACK architecture.

MarkerStack provides a much-loved-and-used function from my prior editor: Multi-Edit. No other Sublime Text package provides this service.

• 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. ***
• [n/a] I use .gitattributes to exclude files from the package: images, test files, sublime-project/workspace.

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