Fix the pointer icon in SelectionContainer

[m-sasha]

In the last big merge from upstream we accidentally removed the pointerHoverIcon from SelectionRegistrar.makeSelectionModifier. But we also gained a mechanism to define the pointer icon from upstream. It didn't, however, specify a pointer icon for any platforms other than Android.

Proposed Changes

• Fix the pointer icon in SelectionContainer by implementing Modifier.textPointerHoverIcon to return pointerHoverIcon(textPointerIcon) on all platforms.
• Set textPointerIcon to PointerIcon.Text on all platforms. This can have addition effects, as it's also used in Modifier.defaultTextFieldPointer and Modifier.cupertinoTextFieldPointer.

Testing

Test: Added a unit test.

[m-sasha]

Added @eymar and @mazunin-v-jb as reviewers because it affects wasmJs and ios platforms.