Add quiet input to suppress info-level log output (#898)

## Summary

Adds a new `quiet` input (default: `false`) that suppresses `info`-level
log output when set to `true`. Only warnings and errors are shown.

Contributes to: #868

src/utils/logging.ts

@@ -0,0 +1,21 @@
+import * as core from "@actions/core";
+
+let quiet: boolean | undefined;
+
+function isQuiet(): boolean {
+  if (quiet === undefined) {
+    quiet =
+      typeof core.getInput === "function" && core.getInput("quiet") === "true";
+  }
+  return quiet;
+}
+
+export function info(msg: string): void {
+  if (!isQuiet()) {
+    core.info(msg);
+  }
+}
+
+export const warning = core.warning;
+export const error = core.error;
+export const debug = core.debug;