Set up dependency caching for playwright GitHub actions workflow

[mickmister]

Caching was disabled in the playwright yaml file as it was causing issues with package-lock.json being cached. The task here is to re-introduce caching of dependencies into the workflow properly.

#236 (comment)

https://github.com/mattermost/mattermost-plugin-todo/blob/1afeeb0804ffa972c07c63725aa9ec883b55603c/.github/workflows/playwright.yml#L108-L114