Use MarshalIndent for better readability (#9)

## Summary

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

README.md

@@ -140,7 +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
+- **[json.Marshaler]:** If your type implements [json.Marshaler], this will be used and the snapshot will be a valid JSON file (using MarshalIndent for readability)
 - **[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

snapshot.go

@@ -77,7 +77,11 @@ func (s *Shotter) Snap(value any) {
 		}
 		current.Write(content)
 	case json.Marshaler:
-		content, err := val.MarshalJSON()
+		// TODO(@FollowTheProcess): If it's JSON, I'd like to make the file extension JSON
+		// for better editor support etc.
+
+		// Use MarshalIndent for better readability
+		content, err := json.MarshalIndent(val, "", "  ")
 		if err != nil {
 			s.tb.Fatalf("%T implements json.Marshaler but MarshalJSON() returned an error: %v", val, err)
 			return

snapshot_test.go

@@ -183,7 +183,7 @@ func TestSnap(t *testing.T) {
 			name:         "json marshaler",
 			value:        jsonMarshaler{},
 			wantFail:     false,
-			existingSnap: `{"key": "value"}`,
+			existingSnap: "{\n  \"key\": \"value\"\n}",
 		},
 		{
 			name:     "json marshaler error",

testdata/snapshots/TestSnap/json_marshaler.snap.txt

@@ -1 +1,3 @@
-{"key": "value"}
+{
+  "key": "value"
+}