feat(strings/uuidv7): add UUIDv7 generation (#10)

The UUID v7 format is a time-ordered random UUID. It uses a timestamp
with millisecond precision in the most significant bits, followed by
random data. This provides both uniqueness and chronological ordering,
making it ideal for database primary keys and situations where sorting
by creation time is desired.

References:
- https://uuid7.com/
- https://www.ietf.org/archive/id/draft-peabody-dispatch-new-uuid-format-04.html#name-uuid-version-7

randsmust/strings_example_test.go

@@ -75,3 +75,8 @@ func ExampleUUID() {
 	s := randsmust.UUID()
 	fmt.Println(s) // => 5baa35a6-9a46-49b4-91d0-9530173e118d
 }
+
+func ExampleUUIDv7() {
+	s := randsmust.UUIDv7()
+	fmt.Println(s) // => 01954a3a-a06f-7186-8774-51a770503eb2
+}