fix(components): [tree-select] case-insensitive when filterable

[hellomrbigshot]

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Description

🤖 Generated by Copilot at 304bdc5

Enhanced the filter functionality and code style of the tree-select component.

Related Issue

Fixes #14613 .

Explanation of Changes

🤖 Generated by Copilot at 304bdc5

• Import escapeStringRegexp function from @element-plus/utils to escape special characters in filter value for tree-select component (link)
• Change filter logic to use case-insensitive regular expression instead of string inclusion check, to improve user experience and match behavior of other components (link)
• Adjust indentation of ternary operators and filter function to follow code style of project (link, link, link, link)

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[github-actions]

[github-actions]

Hello @hellomrbigshot, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=14617
Please comment the example via this playground if needed.

[kooriookami]

LGTM