Make --pretty-json work with Standard JSON output

test/cmdlineTests/pretty_json_indent_only/args

@@ -0,0 +1 @@
+--json-indent 7

test/cmdlineTests/pretty_json_indent_only/input.json

@@ -0,0 +1,10 @@
+{
+	"language": "Solidity",
+	"sources":
+	{
+		"A":
+		{
+			"content": "// SPDX-License-Identifier: GPL-3.0\npragma solidity >=0.0; contract C {}"
+		}
+	}
+}

test/cmdlineTests/pretty_json_indent_only/output

@@ -0,0 +1 @@
+

test/cmdlineTests/pretty_json_indent_only/output.json

@@ -0,0 +1,9 @@
+{
+       "sources":
+       {
+              "A":
+              {
+                     "id": 0
+              }
+       }
+}