Add a clear button to the date picker

`clearable` empties a date, or both ends of a range, closes an open
picker and hands focus back to the field, as the time picker's does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

resources/js/datepicker.js

@@ -366,6 +366,18 @@ document.addEventListener('alpine:init', () => {
             }
         },
 
+        /** The clear button: no date (no start and no end), closed, and focus back in the field. */
+        clear() {
+            if (this.open) {
+                this.cancel(false)
+            }
+
+            this.fieldError = ''
+            this.text = ''
+            this.write(null)
+            this.$refs.input.focus()
+        },
+
         fieldProblem(value) {
             if (!config.range) {
                 return this.problem(value)