Use json.Marshaler over TextMarshaler if it's present (#8)

## Summary

<!-- Describe your changes in detail here, if it closes an open issue,
include "Closes #<issue>" -->

README.md

@@ -140,6 +140,7 @@ The files will be named automatically after the test:
 Because of this, it needs to know how to serialise your value (which could be basically any valid construct in Go) to plain text, so we follow a few basic rules in priority order:
 
 - **`snapshot.Snapper`:** If your type implements the `Snapper` interface, this is preferred over all other potential serialisation, this allows you to have total control over how your type is snapshotted, do whatever you like in the `Snap` method, just return a `[]byte` that you'd like to look at in the snapshot and thats it!
+- **[json.Marshaler]:** If your type implements [json.Marshaler], this will be used and the snapshot will be a valid JSON file
 - **[encoding.TextMarshaler]:** If your type implements [encoding.TextMarshaler], this will be used to render your value to the snapshot
 - **[fmt.Stringer]:** If your type implements the [fmt.Stringer] interface, this is then used instead
 - **Primitive Types:** Any primitive type in Go (`bool`, `int`, `string` etc.) is serialised according to the `%v` verb in the [fmt] package
@@ -155,5 +156,6 @@ This package was created with [copier] and the [FollowTheProcess/go_copier] proj
 [copier]: https://copier.readthedocs.io/en/stable/
 [FollowTheProcess/go_copier]: https://github.com/FollowTheProcess/go_copier
 [fmt]: https://pkg.go.dev/fmt
+[json.Marshaler]: https://pkg.go.dev/encoding/json#Marshaler
 [encoding.TextMarshaler]: https://pkg.go.dev/encoding#TextMarshaler
 [fmt.Stringer]: https://pkg.go.dev/fmt#Stringer

snapshot.go

@@ -8,6 +8,7 @@ package snapshot
 import (
 	"bytes"
 	"encoding"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io/fs"
@@ -75,6 +76,13 @@ func (s *Shotter) Snap(value any) {
 			return
 		}
 		current.Write(content)
+	case json.Marshaler:
+		content, err := val.MarshalJSON()
+		if err != nil {
+			s.tb.Fatalf("%T implements json.Marshaler but MarshalJSON() returned an error: %v", val, err)
+			return
+		}
+		current.Write(content)
 	case encoding.TextMarshaler:
 		content, err := val.MarshalText()
 		if err != nil {

snapshot_test.go

@@ -67,14 +67,14 @@ func (e explosion) Snap() ([]byte, error) {
 // nosnap has no Snap implementation.
 type nosnap struct{}
 
-// textMarshaler is a struct that implements encoding.TextMarshaller.
+// textMarshaler is a struct that implements encoding.TextMarshaler.
 type textMarshaler struct{}
 
 func (t textMarshaler) MarshalText() (text []byte, err error) {
 	return []byte("MarshalText() called\n"), nil
 }
 
-// errMarshaler is a struct that implements encoding.TextMarshaller, but always returns an error.
+// errMarshaler is a struct that implements encoding.TextMarshaler, but always returns an error.
 type errMarshaler struct{}
 
 func (t errMarshaler) MarshalText() (text []byte, err error) {
@@ -88,6 +88,20 @@ func (s stringer) String() string {
 	return "String() called\n"
 }
 
+// jsonMarshaler is a struct that implements json.Marshaler.
+type jsonMarshaler struct{}
+
+func (j jsonMarshaler) MarshalJSON() ([]byte, error) {
+	return []byte(`{"key": "value"}`), nil
+}
+
+// errJSONMarshaler is a struct that implements json.Marshaler, but always returns an error.
+type errJSONMarshaler struct{}
+
+func (e errJSONMarshaler) MarshalJSON() ([]byte, error) {
+	return nil, errors.New("MarshalJSON error")
+}
+
 func TestSnap(t *testing.T) {
 	tests := []struct {
 		value        any    // Value to snap
@@ -165,6 +179,17 @@ func TestSnap(t *testing.T) {
 			wantFail:     false,
 			existingSnap: "String() called\n",
 		},
+		{
+			name:         "json marshaler",
+			value:        jsonMarshaler{},
+			wantFail:     false,
+			existingSnap: `{"key": "value"}`,
+		},
+		{
+			name:     "json marshaler error",
+			value:    errJSONMarshaler{},
+			wantFail: true,
+		},
 	}
 
 	for _, tt := range tests {

testdata/snapshots/TestSnap/json_marshaler.snap.txt

@@ -0,0 +1 @@
+{"key": "value"}