feat: Implement builtin mode for keepassxc

[bakito]

Implements: #4148

[twpayne]

Thanks for this!

Instead of a new set of template functions, I think this different "mode" for opening KeepassXC databases. Currently chemzoi supports "cache-password" and "open" modes. I think this should be a new "builtin" mode.

What do you think?

[bakito]

Thanks for this!

Instead of a new set of template functions, I think this different "mode" for opening KeepassXC databases. Currently chemzoi supports "cache-password" and "open" modes. I think this should be a new "builtin" mode.

What do you think?

Sounds reasonable.
I modified the implementation and added the functions for the new mode.

[twpayne]

Thank you, this is looking excellent, and I will do a full review after the weekend.

Please also check the contributing guide for low-hanging fruit.

But, thank you, this PR is excellent.

[twpayne]

Thank you, this is looking excellent. Once you've made the small changes to the documentation, this should be ready to merge.

[halostatue]

Looks good overall. Just a couple of comments and only one "required" change (for the docs).

[twpayne]

@bakito Thanks for making the changes. Please can you squash everything into a single commit, and then this should be good to go.

[twpayne]

Thank you @bakito for your excellent work and patience here!

[bakito]

@twpayne I introduced a lint issue in the last change. Sorry for that.
#4166 provides a fix