Fix End of Doc Bugs

### Description Fixes a few end-of-document bugs: - `rectForOffset` had an incorrect `!=` instead of a `<` check for the end of the document. - `mouseDown` needed to be looser on finding content runs for the end of documents. Adds test cases that would have caught both these bugs. ### Related Issues N/A ### Checklist <!--- Add things that are not yet implemented above --> - [x] I read and understood the [contributing guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md) as well as the [code of conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md) - [x] The issues this PR addresses are related to each other - [x] My changes generate no new warnings - [x] My code builds and runs on my machine - [x] My changes are all related to the related issue above - [x] I documented my code ### Screenshots N/A