Improve rendering of modifiers in DescriptorRenderer

* Use bold font to render all modifiers
* Render 'external' according to the style guide, right after modality

compiler/testData/diagnostics/testsWithJsStdLib/native/propertyParameter.txt

@@ -1,6 +1,6 @@
 package
 
-public external final class C {
+public final external class C {
     public constructor C(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.String)
     public final val y: kotlin.String
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean