refactor uiCmdSequence into a separate function

[k-yle]

prerequisite for #11999

In that PR I want to reuse the logic that converts:

{  modifiers: ["⌥"], shortcuts: ["+", "-"], separator: "," }

into:

⌥ Option +, -

To test: check that the keyboard shortcuts popup still renders correctly.

[tordans]

To test: check that the keyboard shortcuts popup still renders correctly.

have you done this already? Or is it a hint for reviewers?

Tested in the preview at https://pr-11998--ideditor.netlify.app/#disable_features=boundaries&map=2.00/0.0/0.0&background=Bing so yes, still works.

[k-yle]

have you done this already? Or is it a hint for reviewers?

yes sorry for the poor wording, i did also test this mysefl

[tyrasd]

Looks good to me. But can we please talk about what indentation style to use for new files? I know https://github.com/openstreetmap/iD/blob/develop/CONTRIBUTING.md#javascript has documented the different rule for ES5/ES6 files, but I think that's just silly (especially in this context, as typescript is neither).

As the majority of our source files still use 4-space indentation, why don't we stick to that even for newly added/refactored typescript code? I would leave the existing two-space indented files as they are as long as we don't touch them in a major way. Does that makes sense? If yes, I can update the documentation to reflect it.

[k-yle]

sounds good to me - changed to 4 spaces now