refactor: rename interpreter to runtime and use receiver methods (#39)

## Description

The codebase previously used "interpreter" terminology and standalone functions for expression operations.
This PR modernizes the architecture by renaming to "runtime" and converting operations to receiver methods.

- Rename `pkg/interpreter` to `pkg/runtime`.
- Move `ReduceOnce` to new `pkg/normalorder` package for reduction strategy isolation.
- Convert standalone functions (`Substitute`, `Rename`, `GetFree`, `IsFree`) to receiver methods on concrete expression types.
- Change `Set` from pointer receivers to value receivers for simpler usage.
- Update all references from "interpreter" to "runtime" terminology throughout the codebase.

### Decisions

- Operations like `Substitute`, `Rename`, `GetFree`, and `IsFree` are now methods on the `Expression` interface, implemented by each concrete type (`Variable`, `Abstraction`, `Application`).
- The `normalorder` package isolates the normal-order reduction strategy, allowing future reduction strategies to be added in separate packages.
- `Set` uses value receivers since Go maps are reference types and don't require pointer semantics.

## Benefits

- Cleaner API: `expr.Substitute(target, replacement)` instead of `Substitute(expr, target, replacement)`.
- Better separation of concerns: reduction strategies are isolated from expression types.
- Consistent terminology: "runtime" better reflects the execution model.
- Simpler `Set` usage without needing to manage pointers.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #39
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/emitter/emitter.go

@@ -9,7 +9,7 @@ type Emitter[E comparable] interface {
 }
 
 type BaseEmitter[E comparable] struct {
-	listeners map[E]*set.Set[Listener[E]]
+	listeners map[E]set.Set[Listener[E]]
 }
 
 func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
@@ -41,6 +41,6 @@ func (e *BaseEmitter[E]) Emit(event E) {
 
 func New[E comparable]() *BaseEmitter[E] {
 	return &BaseEmitter[E]{
-		listeners: map[E]*set.Set[Listener[E]]{},
+		listeners: map[E]set.Set[Listener[E]]{},
 	}
 }