Fix bookmarked range duplication bug

[Ben3eeE]

Description of the Change

Changes the DisplayMarkerLayer of a bookmark to use exclusive insertion strategy. This means that insertions at the start or end of the marked range should be interpreted as happening outside the DisplayMarkerLayer and not extend the range. The current behavior of extending the range when typing outside has many points of failure as discussed in https://github.com/atom/bookmarks/issues/62.

Gif of behavior after the change:

Alternate Designs

N/A

Benefits

• Makes bookmarks behave more like sublime.
• Adds a test for this behavior.

Possible Drawbacks

Someone depends on the current behavior.

Applicable Issues

Fixes https://github.com/atom/bookmarks/issues/62

/cc: @as-cii

[as-cii]

Nice work here! Thanks, @Ben3eeE! 💯

[pdonias]

Hi! Does it fix this kind of behaviour?

Thanks for the fix anyway :)

[Ben3eeE]

@pdonias