TextEditor: Specify the starting line and column number using colons

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 b474f49164
Closes #5589

Userland/Applications/TextEditor/CMakeLists.txt

@@ -2,6 +2,7 @@ compile_gml(TextEditorWindow.gml TextEditorWindowGML.h text_editor_window_gml)
 
 set(SOURCES
     main.cpp
+    FileArgument.cpp
     MainWidget.cpp
     TextEditorWindowGML.h
 )