Add ModalFrontstageButton component (backport #1156)

[mergify]

Changes

This PR adds backButton property to ModalFrontstageInfo interface to allow specifying of a custom back button for a modal frontstage. Additionally ModalFrontstageButton component is added to maintain visual consistency between modal frontstages.

ModalFrontstageButton uses iTwinUI IconButton instead of outdated custom implementation.
Additionally, ModalFrontstageButton is used as a default back button if no backButton is provided via ModalFrontstageInfo.

Testing

Added additional story.

---

This is an automatic backport of pull request #1156 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b8a94f9 has failed:

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Changes to be committed:
	modified:   common/api/appui-react.api.md
	modified:   common/api/summary/appui-react.exports.csv
	new file:   common/changes/@itwin/appui-react/modal-frontstage-back_2024-12-13-14-49.json
	modified:   docs/storybook/src/frontstage/Modal.stories.tsx
	modified:   docs/storybook/src/frontstage/Modal.tsx
	modified:   e2e-tests/tests/configurableui/configurable-ui.test.ts-snapshots/configurable-ui-test-1-chromium-linux.png
	modified:   e2e-tests/tests/content/content-layout.test.ts-snapshots/content-layout-test-1-chromium-linux.png
	modified:   e2e-tests/tests/content/split-pane.test.ts-snapshots/content-layout-test-1-chromium-linux.png
	modified:   e2e-tests/tests/frontstage/modal-frontstage.test.ts-snapshots/modal-frontstage-test-1-chromium-linux.png
	modified:   e2e-tests/tests/picker/view-selector.test.ts-snapshots/view-selector-test-1-chromium-linux.png
	modified:   ui/appui-react/src/appui-react.ts
	modified:   ui/appui-react/src/appui-react/framework/FrameworkFrontstages.ts
	modified:   ui/appui-react/src/appui-react/frontstage/ModalFrontstage.scss
	modified:   ui/appui-react/src/appui-react/frontstage/ModalFrontstage.tsx
	renamed:    ui/appui-react/src/appui-react/layout/widget/tools/button/Back.scss -> ui/appui-react/src/appui-react/frontstage/ModalFrontstageButton.scss
	new file:   ui/appui-react/src/appui-react/frontstage/ModalFrontstageButton.tsx
	modified:   ui/appui-react/src/appui-react/widget-panels/ModalFrontstageComposer.tsx
	modified:   ui/appui-react/src/test/frontstage/ModalFrontstage.test.tsx
	deleted:    ui/appui-react/src/test/layout/widget/tools/Back.test.tsx

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/changehistory/NextVersion.md
	deleted by them: ui/appui-react/src/appui-react/layout/widget/tools/button/Back.tsx

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