API to set priorities of individual PluginHandlers

[mrcjkb]

Is your enhancement request related to a problem? Please describe.

To solve #3636, I had to split the hls-pragmas-plugin into three separate plugins (see #3640),
so that I could decrease the priority of the "disable this warning" codeAction.

A cleaner solution would be to have a single plugin with priorities set for the individual PluginHandlers.

Describe the solution you'd like

• A property for each pluginHandler (analogous to the PluginDescriptor.pluginPriority property) that lets us define the priority of each pluginHandler.
• This priority should be used to sort the code actions and completions.

[michaelpj]

Or we could go even further, and return some type from a plugin handler other than the bare LSP response type, and then we could put the priorities into the result type for things that look like "a list of results".

[michaelpj]

Unclear whether this would necessarily be an improvement. At the moment the plugin is our "unit of configurability", and maybe we should just live with that as a coherent design.