fix: use namespace import for @actions/core v3

@actions/core v3.0.0 is a pure ESM package with only named exports.
Change `import core from` to `import * as core from` in all source files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: parkerbxyz

lib/request.js

@@ -1,4 +1,4 @@
-import core from "@actions/core";
+import * as core from "@actions/core";
 import { request } from "@octokit/request";
 import { ProxyAgent, fetch as undiciFetch } from "undici";
 

main.js

@@ -1,6 +1,6 @@
 // @ts-check
 
-import core from "@actions/core";
+import * as core from "@actions/core";
 import { createAppAuth } from "@octokit/auth-app";
 
 import { getPermissionsFromInputs } from "./lib/get-permissions-from-inputs.js";

post.js

@@ -1,6 +1,6 @@
 // @ts-check
 
-import core from "@actions/core";
+import * as core from "@actions/core";
 
 import { post } from "./lib/post.js";
 import request from "./lib/request.js";