Combine snippets with bookmarks

[pokey]

A bookmark should be able to indicate how new elements should be appended to it, kind of like an extreme version of #443. We could combine several of these into a bigger bookmark, so that for example in one place you transform inputs into a list, in another they become a dictionary with a specific form, etc, recreating some of the functionality of https://github.com/plopjs/plop.

A good example of the type of operation we'd like to support is adding a new color to cursorless. It requires boilerplate in many places, and non-boilerplate in a couple. We'd like the user to be able to establish a smart cross-file bookmark / snippet that can generate all the boilerplate needed, and guide the user to the places where there is non-boilerplate to be filled out (eg the actual color hex)

See also:

• Stable snippets #443
• Smarter snippet holes #445
• Add "mark" action to bookmark a target #46