This allows the user to specify a specific line and column number to
start at when opening a file in TextEditor through the terminal, by
adding a colon after the file name.
For example, `TextEditor ReadMe.md:10:5` will open ReadMe.md and put
the cursor on line 10 at column 5.
To ensure that the user isn't trying to open a file that actually has
colons in its name, it checks if the file exists before parsing.
Replaces the feature added in b474f49164Closes#5589
