WIP: fix: add W3C KeyboardEvent.key values support

[ronnymikalsen]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalLibraryOfNorway/ngx-mime/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?
KeyboardEvent.keyCode is deprecated, also MDN recommends to avoid it for new projects in favor of KeyboardEvent.key.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

Issue Number:
Fixes #227

Does this PR introduce a breaking change?

[ ] Yes
[X] No