build: set up token extraction for the timepicker

Adds a target to extract the tokens from the timepicker.

Author: crisbeto

src/material/timepicker/BUILD.bazel

@@ -1,5 +1,6 @@
 load(
     "//tools:defaults.bzl",
+    "extract_tokens",
     "markdown_to_html",
     "ng_module",
     "ng_test_library",
@@ -72,6 +73,11 @@ markdown_to_html(
     srcs = [":timepicker.md"],
 )
 
+extract_tokens(
+    name = "tokens",
+    srcs = [":timepicker_scss_lib"],
+)
+
 filegroup(
     name = "source-files",
     srcs = glob(["**/*.ts"]),