feat: truncate error string if it is too long

We use esbuild to transform ts files to js ones. When handling
unexpected error messages, it may print the transpiled minified js file
code. This is meaningless, so if the stringified error is too long,
truncate it.

src/error.ts

@@ -49,7 +49,11 @@ export function handleError(error: unknown): never {
   }
 
   // Unknown error. This may be a bug of this action.
-  core.debug(stringify(error))
+  let str_err = stringify(error)
+  if (str_err.length > 256) {
+    str_err = `${str_err.slice(0, 256)} <truncated>`
+  }
+  core.debug(str_err)
   if (error instanceof Error) {
     core.setFailed(`Unknown error occurred: ${error.message}`)
   } else {