Add command to freeze navigation map

[pokey]

• Takes a target, indicating what section of the document to freeze, so can say eg "freeze funk air"
• Any hats within that region will stay the same until unfrozen / refreshed
• If new text is added within that region, it will be frozen once its hat is allocated
• How to handle undo / redo?
• How to handle deletion? Do we just remove tokens that become empty? (that's what happens to deleted tokens; they become an empty range)
• When adding decorations, hatTokenMap will pass in a list of frozen tokens
• addDecorations will use those tokens as start of new hat token map
• addDecorations will not decorate tokens that are contained within an existing token range. For tokens that partially intersect an existing token, I guess it will decorate the part that is not intersecting the existing token range
• addDecorations will not use any decorations that are already taken by a frozen token
• Add command to "refresh" a target area. That will refresh any frozen section containing that target with new hats, but leave the area frozen
• Add command to "unfreeze" target. That will unfreeze any frozen section containing that target
• Add command to unfreeze all
• Add indicator in status bar that there are frozen tokens
• Decorate the frozen section with subtle blue transparent background color
• Preserve frozen tokens even when editor goes out of view, so that can be used as basic version of bookmarks

The advantage of this approach over #432 is that it gives something akin to a lightweight bookmark / stable snippet