API to set priorities of individual PluginHandlers

## 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.

Start by tracing the existing PluginDescriptor.pluginPriority implementation and the code paths that sort code actions and completions. Done means individual PluginHandlers can define priorities and those priorities affect the ordering of both result types, with coverage for the new behavior.

Written by the indexing model from the issue text.