[DAP] Add --dap-port as a command line argument

[van800]

This is very similar to --lsp-port, which was added in #81844

I have started a conversation in https://chat.godotengine.org/channel/devel?msg=qEJuhGNKqXKsxmjZr, however haven't got responses so far.

The PR solves a case, when 2 GodotEditors are opened with projects and external editor tries to use one of the editors for debugging via DAP via Godot Editor (DebugAdapterServer, default port 6006).
In Rider already can start GodotEditor with LSP server in a headless mode, and this PR would help to use such GodotEditor also for debugging. Other external editors, which use DAP may also benefit from the PR.

[van800]

@akien-mga what do you think?

Also I have a question out of scope of this PR.
I have noticed that vscode has its own way to connect directly to the game without using the Editor, is it for historical reasons or is it considered to be more strait-forward usable scenario? If it provides premium usability, why not move DAP server from the Editor to the Engine and remove the necessity to have the Editor running?

[akien-mga]

Thanks!