Add ability to set `editorHeight`

[gadenbuie]

In some layouts, in particular the vertical layout, the entire size of the embedded shinylive component is not completely controlled by viewerHeight. For long apps, it would be helpful to be able to limit the number of lines shown, either via an option like editorHeight or editorLines.

[gadenbuie]

I've run into this again and having a long R script (e.g. a single app.R with more than 25 lines) makes shinylive basically unusable in a Quarto page.

I'd recommend we do both:

• Add editorLines (or editorHeight, whichever is easier) to control the height of the editor. Useful in any mode, but in particular in the vertical layout where the editor height is unbounded.

• If we implement it as editorHeight, then vertical mode should either be driven by viewerHeight or should use min(viewerHeight, editorHeight).