[hack] Add some range calculations to Lsp_helpers
[hack] Add some range calculations to Lsp_helpers

Summary: Here are some range calculations. They add up to the algorithm "if there's a squiggle, and the user performs a DidChange event,  then what's the new range for the squiggle?"

Reviewed By: pakoito

Differential Revision: D9948098

fbshipit-source-id: 8fa0edd23d46e06c03965f1f93ac4e643e8712ed

popham/flow,jamesgpearce/flow,popham/flow,jamesgpearce/flow,AgentME/flow,samwgoldman/flow,facebook/flow,mroch/flow,gabelevi/flow,popham/flow,gabelevi/flow,facebook/flow,mroch/flow,popham/flow,jamesgpearce/flow,jamesgpearce/flow,jamesgpearce/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,gabelevi/flow,popham/flow,mroch/flow,samwgoldman/flow,facebook/flow,AgentME/flow,jamesgpearce/flow,AgentME/flow,samwgoldman/flow,AgentME/flow,facebook/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,gabelevi/flow,gabelevi/flow,AgentME/flow,jamesgpearce/flow,samwgoldman/flow,popham/flow,mroch/flow,popham/flow