Allow to provide a custom placeholder

[tcitworld]

Is your feature request related to a problem? Please describe.
When using the editor from another app with OCA.Text.createEditor, I can't set the default value for the placeholder

Describe the solution you'd like
The Placeholder extension should use a placeholder option in RichText.js, which can be passed as a prop to Editor/MarkdownContentEditor and finally exposed to createEditor().

Describe alternatives you've considered
Adding my own configured instance of the Placeholder extension, if #5550 is available.

Or possibly directly changing the extension's configured value for the placeholder, if possible after the editor has loaded?

[juliushaertl]

I think providing a custom placeholder sounds like a good idea, for the generic ability to provide extensions I'd be more skeptical