Add advancedUsage to standard layout of ToolbarItem

[GerardasB]

Changes

This PR adds an optional advancedUsage property to the StandardLayoutToolbarItem interface. It takes precedence over standard toolbar usages described in ToolbarUsage enum. The new property type is extensible: allows custom values to be used and new string literal types defined in the union type.

"view-settings" advanced usage type is introduced to display toolbars in the bottom-right area of the content.

Known issues

• toolbar alignment when adjacent vertical toolbar contains no items (i.e. view-navigation horizontal toolbar offset when vertical toolbar has no items) https://itwin.github.io/appui/1474/?path=/story/frontstage-toolbars--all-custom&args=hideNavigationAid:!true;viewNavigationVerticalLength:0 Fix toolbar alignment w/ empty toolbars
• toolbar overflow in a shared container (i.e. view-navigation and view-settings vertical toolbars share the same container and should use all available space, instead of 50/50) https://itwin.github.io/appui/1474/?path=/story/frontstage-toolbars--all-custom&args=hideNavigationAid:!true;viewSettingsVerticalLength:10 NOTE: this may be needed if multiple button groups are used for grouping, instead of current dividers.
• Inconsistent spacing due to flex/grid gaps Make spacing from the edge of the viewport consistent #713
• Overflow logic not applied to horizontal view-settings toolbar (toolbars are rendered outside of viewport bounds) https://itwin.github.io/appui/1474/?path=/story/frontstage-toolbars--all-custom&args=viewSettingsHorizontalLength:25 Secondary horizontal toolbar to take at most 50% of width
• Redundant spacing in horizontal view-settings toolbar when navigation aid is displayed https://itwin.github.io/appui/1474/?path=/story/frontstage-toolbars--all&args=hideNavigationAid:!false Move navigation aid to horizontal container

Preview

https://itwin.github.io/appui/1474/?path=/story/frontstage-toolbars--all