feat(material/core): add date format for month label (#21044)

Author: MichaelJamesParsons

src/material/core/datetime/date-formats.ts

@@ -15,6 +15,7 @@ export type MatDateFormats = {
   },
   display: {
     dateInput: any,
+    monthLabel?: any,
     monthYearLabel: any,
     dateA11yLabel: any,
     monthYearA11yLabel: any,