feat: add Decoder.UseNumber and the Number type

Assisted-by: GLM 5.3 Flash

parser.go

@@ -36,6 +36,10 @@ type parser struct {
 	maxDepth int
 	depth    int
 
+	// useNumber leaves the numbers a Number carries the literal, instead of
+	// the evaluated int64 or float64 the tree holds by default.
+	useNumber bool
+
 	root    map[string]any
 	current map[string]any
 	headers map[string]bool
@@ -721,6 +725,11 @@ func (p *parser) parseAtom() (any, error) {
 	if err != nil {
 		return nil, p.errf("%s", err)
 	}
+	// The token's shape is validated either way; UseNumber only keeps the
+	// literal instead of the evaluated value.
+	if p.useNumber {
+		return Number(tok), nil
+	}
 	return v, nil
 }