[split-view]: update label attribute to be customisable

[najikahalsema]

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Description of issue

Follow up to the work done in https://github.com/adobe/spectrum-web-components/pull/3800

The this.label attribute in sp-split-view is inconsistent with our other this.label attributes, because it sets the value internally to the code like so:

const label = this.label || this.resizable ? 'Resize the panels' : undefined;

The aria-label is set to label if it is defined, rather than this.label. Do we want this to be customisable by the user and have "Resize the panels" as a fallback option? Or is this fine as it is?

Reference: "The splitter also has an accessible name that matches the name of the primary pane."