Bugfixes for the beta.5 bind-invokable refactor

[dfreeman]

The 1.0.0-beta.5 release of Glint introduced support for {{helper}} and {{modifier}} by way of a new bind-invokable abstraction based on the existing {{component}} implementation. As folks have kicked the tires on that refactor, a few bugs and inconsistencies have come to light:

• {{modifier}} fails when pre-binding multiple positional arguments #559: the type signature allowed for multiple arguments, but due to some strange interactions in TypeScript's context-sensitive inference, it would frequently infer a version of the signature that only accepted one positional arg. We fix this by wrapping the resolveForBind call in an IIFE, which acts as an opaque barrier to context propagation for inference.
• Difference between using component and assigning it via let #556: we weren't constraining the results of bind-invokable calls with emitContent as we should have when they appeared in a top-level position
• component "link-to" seems broken on beta.5 #555: we were missing a special case for string-named DirectInvokables for environment-ember-loose

I've run some targeted sets of tests with these changes, but will let CI tell me if other further-reaching impacts need to be accounted for somewhere in our integration/acceptance tests.