[@mantine/core] Tree: Add optional initialCheckedState property

[fredgig]

Motivation: This PR aims to add an optional initialCheckedState prop to useTree, akin to initialExpandedState and initialSelectedState, which addresses #6641.

• Adds initialCheckedState property to useTree hook;
• Adds default property to checkedState hook;
• Renames internal getInitialState function to getInitialExpandedState;
• Adds internal getInitialCheckedState function: This takes the current tree checkedState and ensures only leaf nodes are present, which allows users to input parent nodes in the initialCheckedState property as it outputs their leaf nodes;
• Adds setCheckedState instruction and dependency to initialize callback to update checkedState with behaviour indicated in previous point.

[rtivital]

Thanks!