Update ESLint & Prettier. Modernize configs. Run Prettier.

[richard-viney]

This PR contains two commits:

• The first commit updates the versions and configs for ESLint and Prettier to latest, and adds a "format" npm script.
• The second commit runs prettier on the code.

This reduces friction while working on the dcmjs code because all files are already formatted by Prettier which means you don't run into situations where you open a file, press save on a small edit, and have lots of other unrelated things change in the file due to prettier not having been run consistently.

One downside here is if merged it might cause conflicts with other PRs, although they should be easily addressed if that's the case.

Thoughts?

[pieper]

This sounds good to me, but I agree there's an impact on existing PRs. Anyone else have an opinion here? @swederik @sedghi @Punzo ?

[sedghi]

I don't have an open PR, but there is always an open PR, and the person can rebase IMO

[Punzo]

No issue on my side.