[image-optimizer] Refactor into lightweight transform module

[styfle]

This PR is strictly a refactor, no logic should change.

The idea is that we eventually want to run the image optimizer transform step in a child process. This PR refactors the code to make a new transform.ts that is lightweight and should reduce some of the overhead of loading a large module graph when spawning a new process.

[github-actions]

Failing test suites

Commit: e2635d8 | About building and testing Next.js

pnpm test-dev test/e2e/app-dir/actions/app-action-node-middleware.test.ts (job)

• app-dir action handling > HMR > should support updating the action (DD)

Expand output

● app-dir action handling › HMR › should support updating the action

expect(received).toBeGreaterThan(expected)

Expected: > 1000
Received:   0

  1052 |             await browser.elementByCss('#inc').click()
  1053 |             const val = Number(await browser.elementById('count').text())
> 1054 |             expect(val).toBeGreaterThan(1000)
       |                         ^
  1055 |           })
  1056 |         } finally {
  1057 |           await next.patchFile(filePath, origContent)

  at toBeGreaterThan (e2e/app-dir/actions/app-action.test.ts:1054:25)
  at retry (lib/next-test-utils.ts:871:14)
  at Object.<anonymous> (e2e/app-dir/actions/app-action.test.ts:1051:11)