Convert `--is-collection` and `--collection-flatmap` to `--flag`/`--no-flag`-style boolean flags, add `--no-strict`

[felixfontein]

Ref: #199
Ref: #196 (comment)

[gotmax23]

Re your comment in #196: The issue with BooleanOptionalAction is that it tracebacks when default is argparse.SUPPRESS but help is set to something. I would rather vendor BooleanOptionalAction than raise the minimum Python version. Also, I think setting requires-python to something like >=3.10.3 will cause issues with tools like poetry when a project sets its minimum python to "^3.10" because not all patch versions of 3.10 are included in the range and it'll fail to resolve/choose an older version.