unify offcanvas keydown event logic with modal

[Ronid1]

Description

Since the offcanvas keydown event listener behaves like the modal keydown listener, the former was refactored like the modal. Making it more readable by using positive conditions instead of negative ones.

Motivation & Context

To make refactor and bugfix separated, a new PR was opened as discussed in this comment.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

N/A

Related issues

Splited from #37968

[Ronid1]

@GeoSot updated according to your review. Rebased and squashed.

[GeoSot]

It looks fin to my eyes.
Either way, we didn't have to use event.preventDefault() and was not tested too

note: I suppose we don't need any tests for this