feat(ui5-flexible-column-layout): add arrow icon functionality

[NakataCode]

• Added the icon in the FCL tsx template(positioned in the first separator)
• Added styling for the arrow button
• Added functionality for the arrow button, so we can toggle the first column switchLayoutOnArrowPress
• The icon property of the button dynamically chooses an arrow icon based on the startArrowDirection property.
• Added two new layouts ThreeColumnsStartHiddenMidExpanded and ThreeColumnsStartHiddenEndExpanded
• Added keyboard handling for the arrow, when Enter or Space is pressed on the button, the _onArrowKeydown function is triggered
• Added keyboard handling for left and right key on the arrow
• Added new sample in the test page
• Added tests

JIRA: BGSOFUIPIRIN-6623

[kineticjs]

also noticed accidentally that onSeparatorPress is triggered when we press an arrow. Did not seem like a big issue but we can add e.g:

if (e.target as HTMLElement === this.startArrowDOM) {
	return;
}

to the beginning of onSeparatorPress to prevent it starting an obsolete separatorMovementSession upon arrow-click

[ui5-webcomponents-bot]

🎉 This PR is included in version v2.7.0-rc.1 🎉

The release is available on v2.7.0-rc.1

Your semantic-release bot 📦🚀