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/FileArgument.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2021, ry755 <ryanst755@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/String.h>
+
+class FileArgument final {
+public:
+    FileArgument(String);
+    ~FileArgument();
+
+    String file_name() { return m_file_name; }
+    Optional<size_t> line() { return m_line; }
+    Optional<size_t> column() { return m_column; }
+
+private:
+    String m_file_name;
+    Optional<size_t> m_line;
+    Optional<size_t> m_column;
+};