Throw a clear error when a callback is passed as the first argument of configureImageRule()/configureFontRule()

[Amoifr]

Q	A
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	Fix #1221
License	MIT

configureImageRule() and configureFontRule() take (options, ruleCallback). When a callback is passed as the first argument, for example:

Encore.configureFontRule(rule => {
    rule.oneOf[1].generator.filename = 'whatever/[name][ext]';
});

the function lands in the options parameter. Object.keys(aFunction) is empty, so no option validation fails, and the callback is silently dropped: the rule is never customized. This is a common mistake because several other Encore.configure*() methods take a single callback.

This PR turns that silent no-op into a helpful error pointing at the correct 2-argument form:

configureFontRule() expects an options object as its first argument. To pass only a callback, use configureFontRule({}, callback).

Only broken calls are affected (a function is never a valid options value), so there is no BC impact on working code.

This addresses the most surprising part of #1221 (the callback being ignored without any feedback). Note that overriding the filename does work today through the documented 2-argument callback by modifying the nested rule.oneOf[1].generator; setting rule.generator at the top level has no effect because Encore wraps the asset rule in a oneOf. Happy to follow up on that ergonomics point separately if you think it is worth improving.