feat: add working-directory input

[dsherret]

Adds a working-directory input to run dprint check in a subdirectory, such as a package in a monorepo:

- uses: dprint/check@v2
  with:
    working-directory: packages/app

• The config-path and args inputs are relative to the working directory, while the annotations and the unformatted-files output stay relative to the workspace since that's what GitHub needs.
• The cache key's config hash joins the working directory and config path, since hashFiles is relative to the workspace.
• CI checks tests/working-dir, which has its own config and a formatted file, after the poorly-formatted file is created at the repo root. The check would fail if it didn't run in the directory, and it asserts unformatted-count is 0.