fix: improve cross platform compatibility

[s-h-a-d-o-w]

Currently, e.g. playwright-watch exec yarn playwright test doesn't work on Windows.

Resolved through by spawning commands in a shell on Windows (See also: https://nodejs.org/docs/latest-v14.x/api/child_process.html#child_process_spawning_bat_and_cmd_files_on_windows )

Also did the following to allow for contributions to this project by Windows users:

• Remove single quotes in the lint script
• Remove redundant prettier execution (This is already done by eslint anyway, through the prettier plugin)

(Plus: There were multiple lockfiles. And the one for yarn was outdated.)