ci: reject if unexpected large files are committed

[alxndrsn]

Noted while looking into #1572

Ref #672

What has been done to verify that this works as intended?

Run locally, run in frontend and backend repos. Only fails in backend, which has a big package-lock.json.

Why is this the best possible solution? Were any other approaches considered?

An alternative approach would be that used in central-backend's CircleCI job:

https://github.com/getodk/central-backend/blob/97981e9694f392e5b3df90a2d28c6f7d3aefef17/.circleci/config.yml#L16-L19

Advantages of this version:

• only checks committed files, so
  • much faster (<10ms vs 1+ second)
  • no false-positives - running the central-backend script locally fails for node_modules and generated test files
• hopefully more readable (even if awk may not be familiar)

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No effect.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

Follow-up

If this is accepted, it could be copied to odk-central-frontend to close #672.