Suggest installing types as dev dependencies

[ccouzens]

Typescript types are usually only required at build time, and not runtime. But the old instructions would end up with the types being around at run-time.

From the npm install documentation

npm install saves any specified packages into dependencies by default.

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.