Remove use of non-standard `caretRangeFromPoint` from Coda

Currently, Firefox Mobile reports as not supported by Coda, and that’s because Coda uses a non-standard function: Document: caretRangeFromPoint() method - Web APIs | MDN

Bug related to this has been open for 5 years now: 1550635 - window.document.caretRangeFromPoint is not supported in Firefox

Can usages of this be removed from the Coda codebase so that Coda follows web standards?