Add Extension point for plugins to be able to contribute theming requirements (e.g. source code highlighting)

[timja]

What feature do you want to see added?

Discussed this a bit with @uhafner in a couple of places.

The https://github.com/jenkinsci/prism-api-plugin has added it's own theming component to choose the prism theme.

This isn't ideal as:

1. The users code highlighting theme isn't adapted when their main theme is
2. The code highlighting theme is in a completely different place

It would be ideal if there was an extension point that plugins could implement adding to the UI of this plugin.
It would also be ideal if the Theme extension point added a Map<String,String> properties method where themes could contribute information to these extensions.

e.g.

Dark theme could set a property "prismTheme" = "dark"

Prism API could then default to that.

Then Prism API would add an optional property Override source code highlighting theme which would allow the user to select a different one.

Any thoughts @uhafner or others (@janfaracik fyi)?

This would be useful for jenkinsci/design-library-plugin#12

Upstream changes

probably should implement #81 first

[timja]

This could also be used to overwrite codemirror theme so that theme plugins don't need to supply their own

[uhafner]

I think we have two different variants to implement that.

1. Each theme defines a boolean property DARK or LIGHT. The prism plugin reads that value and selects the theme accordingly.
2. We add an extension point (key value pairs?) and the prism plugin implements that extension point for "io.jenkins.prism.theme" and uses the value of that key to select the theme. This allows creating additional themes that use a totally different prism theme as well.

While the former one is simpler to implement, the latter one seems to be more flexible. However, that means each theme author then needs to know, which other plugins exist that provide theming options.

[timja]

While the former one is simpler to implement, the latter one seems to be more flexible. However, that means each theme author then needs to know, which other plugins exist that provide theming options.

Should probably be a section in the README that plugins that implement the extension point should add a reference in.

[timja]

This was done