Add File Commands (#571)

* Add File Commands

* pr updates w/ feedback

* run format

* fix lint/format

* slight update with an example in the docs

* pr feedback

packages/core/src/file-command.ts

@@ -0,0 +1,24 @@
+// For internal use, subject to change.
+
+// We use any as a valid input type
+/* eslint-disable @typescript-eslint/no-explicit-any */
+
+import * as fs from 'fs'
+import * as os from 'os'
+import {toCommandValue} from './utils'
+
+export function issueCommand(command: string, message: any): void {
+  const filePath = process.env[`GITHUB_${command}`]
+  if (!filePath) {
+    throw new Error(
+      `Unable to find environment variable for file command ${command}`
+    )
+  }
+  if (!fs.existsSync(filePath)) {
+    throw new Error(`Missing file at path: ${filePath}`)
+  }
+
+  fs.appendFileSync(filePath, `${toCommandValue(message)}${os.EOL}`, {
+    encoding: 'utf8'
+  })
+}