fix: ignore case when matching function name

[notauserx]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

Update logic to search by the lowercase of func_name. Uses unicode crate to facilitate searching function name ignoring case

fixes: #16737

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

---

This change is 

[TCeason]

LGTM

[notauserx]

Does it make sense to add ignore case search in is_builtin_function(func_name) as well?

[sundy-li]

Let's add a test in name_hit.test to cover the issue.

[notauserx]

Let's add a test in name_hit.test to cover the issue.

Added a test

[BohuTANG]

Hi @notauserx You can continue after fixing conflicts, we're nearly at the end :)

[BohuTANG]

@notauserx Thanks for your contribution.