Resolve plugin[mappings] prefix value at initialization time

Plugins that provide default mappings generally configure a default mapping prefix like `=` and then take a boolean value for `plugin[mappings]` as a shortcut for that prefix. For instance

``` vim
Glaive codefmt plugin[mappings]
```

is a shortcut for

``` vim
Glaive codefmt plugin[mappings]='='
```

But in the first shortcut form the default prefix isn't introspectable (`:Glaive codefmt` shows a value of `{'mappings': 1}` instead of `{'mappings': '='}`). We should add a hook to resolve it immediately so it's introspectable.

Related: When overriding the `plugin[mappings]` value, we could immediately create new mappings at that prefix, and possibly clean up any old ones we can detect as automatically created by the plugin at the old prefix.