Introduce {{helper}} and {{modifier}} helpers

[dfreeman]

High Level

The headline here is adding {{helper}} and {{modifier}}, but the bulk of the diff is actually internal changes to improve inference characteristics for those two helpers and the existing {{component}} one.

Note that the types for these helpers are intrinsically super complicated because their runtime behavior is super complicated, and we're unlikely to ever get 100% fidelity. The error messages can also be heinous, but we provide hints where possible, and this change isn't making matters any worse than they already were on that front with {{component}}.

Details

I had a whole novel written up here and accidentally ⌘-Red, so... 😭

The abridged version is that this PR adds a new DSL method resolveForBind as a sibling to resolve and resolveOrReturn. In concert with that, it introduces a bind-invokable special form. When we encounter a helper declared to be bind-invokable, we treat its first argument specially and hand it off to resolveForBind, which has two effects:

• environments no longer need a custom implementation of {{component}} (and now {{helper}} and {{modifier}}) to account for different resolution semantics
• the actual types for {{component}} (and now {{helper}} and {{modifier}}) can function just in terms of Glint's internal function representation of invokables, not needing to juggle both the translation and the binding semantics

Other Notes

This also adds a WithBoundPositionals utility type as a counterpart to WithBoundArgs, and as part of documenting that I pulled out the Ember-specific "Contextual Components" docs page into a more general "Glint Types" page that describes the user-facing types that @glint/template exposes.

There's some slightly broader reorganization of the docs that I think could be helpful, but I'll tackle that separately from this change.

Closes #412; fixes #410.