fix(material/datepicker): add i18n strings

Add i18n strings for "Start Date" and "End Date". Add `formatYearRangeLabel` to format a year of
dates in a screen reader friendly way (e.g. "2019 to 2020"). The existing method, `formatYearRange`,
uses a dash character which does not read well on screen readers.

These three new strings are not used in this commit. They will be used in future commits as aria
labels and descriptions to fix accessibility bugs.

Author: zarend

src/material/datepicker/datepicker-intl.ts

@@ -51,8 +51,19 @@ export class MatDatepickerIntl {
   /** A label for the 'switch to year view' button (used by screen readers). */
   switchToMultiYearViewLabel: string = 'Choose month and year';
 
-  /** Formats a range of years. */
+  /** A label for the first date of a range of dates (used by screen readers). */
+  startDateLabel?: string = 'Start date';
+
+  /** A label for the last date of a range of dates (used by screen readers). */
+  endDateLabel?: string = 'End date';
+
+  /** Formats a range of years (used for visuals). */
   formatYearRange(start: string, end: string): string {
     return `${start} \u2013 ${end}`;
   }
+
+  /** Formats a label for a range of years (used by screen readers). */
+  formatYearRangeLabel?(start: string, end: string): string {
+    return `${start} to ${end}`;
+  }
 }

tools/public_api_guard/material/datepicker.md

@@ -525,14 +525,17 @@ export class MatDatepickerIntl {
     calendarLabel: string;
     readonly changes: Subject<void>;
     closeCalendarLabel: string;
+    endDateLabel?: string;
     formatYearRange(start: string, end: string): string;
+    formatYearRangeLabel?(start: string, end: string): string;
     nextMonthLabel: string;
     nextMultiYearLabel: string;
     nextYearLabel: string;
     openCalendarLabel: string;
     prevMonthLabel: string;
     prevMultiYearLabel: string;
     prevYearLabel: string;
+    startDateLabel?: string;
     switchToMonthViewLabel: string;
     switchToMultiYearViewLabel: string;
     // (undocumented)