refactor: extract shared token package (#46)

## Description

Both the `saccharine` and `lambda` packages need tokenizing and parsing primitives.
This PR extracts shared token infrastructure into a new `pkg/token` package, then wires both languages up to use it.

- Add `pkg/token` with a generic `Token[T]` type, `Scan`, `ScanAtom`, `ScanRune`, `ScanCharacter`, `IsVariable`, `ParseRawToken`, and `ParseList`.
- Refactor `pkg/saccharine` to delegate to `pkg/token`, removing duplicated scanning and parsing helpers.
- Implement `Codec.Decode` for `pkg/lambda` (scanner + parser) using the shared token package.
- Add `iterator.While` for predicate-driven iteration.
- Rename `iterator.Do` to `iterator.Try` to better describe its rollback semantics.

### Decisions

- The `Type` constraint (`comparable` + `Name() string`) keeps the generic token flexible while ensuring every token type can produce readable error messages.
- `iterator.Do` was renamed to `iterator.Try` since it describes a try/rollback operation, not a side-effecting "do".

## Benefits

- Eliminates duplicated token, scanning, and parsing code between languages.
- Enables the `lambda` package to decode (parse) lambda calculus strings, which was previously unimplemented.
- Makes it straightforward to add new languages by reusing `pkg/token` primitives.

## Checklist

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

Reviewed-on: #46
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/saccharine/token.go

@@ -1,6 +1,10 @@
 package saccharine
 
-import "fmt"
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/token"
+)
 
 // A TokenType is an identifier for any token in the Saccharine language.
 type TokenType int
@@ -21,7 +25,7 @@ const (
 	TokenAssign
 	// TokenAtom denotes an alpha-numeric variable.
 	TokenAtom
-	// TokenSlash denotes the '/' token.
+	// TokenSlash denotes the '\\' token.
 	TokenSlash
 	// TokenDot denotes the '.' token.
 	TokenDot
@@ -29,24 +33,6 @@ const (
 	TokenSoftBreak
 )
 
-// A Token in the Saccharine language.
-type Token struct {
-	Column int       // Where the token begins in the source text.
-	Type   TokenType // What type the token is.
-	Value  string    // The value of the token.
-}
-
-// NewToken creates a [Token] of the given type at the given column.
-// The token's value is derived from its [TokenType].
-func NewToken(typ TokenType, column int) *Token {
-	return &Token{Type: typ, Column: column, Value: typ.Name()}
-}
-
-// NewTokenAtom creates a [TokenAtom] with the given name at the given column.
-func NewTokenAtom(name string, column int) *Token {
-	return &Token{Type: TokenAtom, Column: column, Value: name}
-}
-
 // Name returns the type of the TokenType, as a string.
 func (t TokenType) Name() string {
 	switch t {
@@ -75,7 +61,5 @@ func (t TokenType) Name() string {
 	}
 }
 
-// Name returns the type of the Token, as a string.
-func (t Token) Name() string {
-	return t.Type.Name()
-}
+// Token is the concrete token type for the Saccharine language.
+type Token = token.Token[TokenType]