Add accessibility label for splitview divider

[OskarKruger]

Description

Added a label for screenreader to splitview divider.
According to Aria documentation, both valuenow and valuetext is required.

Plan to add localiztion for this text also, unsure how to combat this when split-panel-element extends from LitElement instead of UmbLitElement. Would love to hear feedback on both points :)

[iOvergaard]

@OskarKruger

Plan to add localiztion for this text also, unsure how to combat this when split-panel-element extends from LitElement instead of UmbLitElement.

You can just change it to extend from UmbLitElement directly. Then you have access to localization.

[OskarKruger]

Thank you for that tip @iOvergaard, it worked like a charm!

I added the localization with the help of UmbLitElement's implementation, and updated most commonly used lang files.
Using NVDA screenreader, this works 👍🏼

[iOvergaard]

LGTM