feat(unity): open scripts from external editor arguments (#2296)

* feat(app): support Unity external editor navigation

* docs(unity): explain manual external editor setup

docs/unity-external-editor.md

@@ -0,0 +1,42 @@
+# Open scripts from Unity
+
+OpenIDEA can open scripts and Console source locations without a Unity plugin.
+
+1. In Unity, open **Edit > Preferences > External Tools** (on macOS,
+   **Unity > Settings/Preferences > External Tools**).
+2. Under **External Script Editor**, choose **Browse** and select OpenIDEA's
+   executable (or its `.app` on macOS).
+3. Set **External Script Editor Args** to:
+
+   ```text
+   $(ProjectPath) --file $(File) --line $(Line) --column $(Column)
+   ```
+
+Unity 2022.3 quotes the expanded project and file paths itself, including paths
+with spaces. Do not add another pair of quotes around these placeholders.
+
+Double-click a script in the Project window or a source location in the Console.
+OpenIDEA opens the project and file, or focuses its existing project window, then
+places the caret at the supplied location. Other open tabs stay in place.
+
+The project argument makes cold launches open the Unity project rather than a
+standalone file. Without it, `--file` opens in an existing matching project window
+or in LightEdit. Relative file paths resolve against the supplied project folder,
+or against the launch working directory when no project is supplied.
+
+For a direct invocation of the application executable, quote paths as required
+by your shell. The separately installed `openidea` command has its own syntax:
+
+```text
+/path/to/openidea "/path/to/My Game" --file "/path/to/My Game/Assets/Player.cs" --line 12 --column 5
+```
+
+`--file=...`, `--line=...` and `--column=...` also work. Lines and columns are
+1-based; Unity's `0` and `-1` mean an unknown position. An omitted or unknown
+column uses the start of the line. Without a known line, the file opens without
+an explicit caret position. Invalid arguments and missing files report an error.
+
+On Windows, use the same permissions for Unity and OpenIDEA when possible.
+If a running instance does not receive a request, restart both applications with
+matching permissions and retry. This integration opens local projects; it does
+not generate C# project files or attach a debugger.