[frameworks] add pnpm to framework install placeholders (#7587)

Add `pnpm` commands to framework `installCommand.placeholders` so that it appears in the UI.

Additionally, adds the `pnpm-lock.yaml` file to an ignore list like the other lock files.

packages/static-build/src/index.ts

@@ -694,6 +694,7 @@ export const build: BuildV2 = async ({
             'node_modules/**',
             'yarn.lock',
             'package-lock.json',
+            'pnpm-lock.yaml',
             'package.json',
             '.vercel_build_output',
           ];