[Panel & Flow Item] Add `"content-top"` slot

[geospatialem]

Check existing issues

• I have checked for existing issues to avoid duplicates

Description

Add a new slot to the panel and flow-item components - "content-top" below the action-bar slot so we can render in a new Dialog component using the "sticky" functionality currently provided in the Modal component.

Acceptance Criteria

1. Slot should be sticky as it is Modal
2. Slot should come with overridable padding
3. The following css var should be added to match Modal's current overrides:
   --calcite-panel-content-top-padding: Specifies the padding of the component's content-top slot

Relevant Info

A dependency needed for the Dialog + Modal component effort in #7886
See Figma file for specs

Which Component

panel
flow-item

Example Use Case

No response

Priority impact

p2 - want for current milestone

Calcite package

• @esri/calcite-components
• @esri/calcite-components-angular
• @esri/calcite-components-react
• @esri/calcite-design-tokens
• @esri/eslint-plugin-calcite-components

Esri team

Calcite (design)