Refactor: consolidate string utilities

[bobzhang]

Summary

• consolidate surrogate pair utilities into string/view.mbt
• remove now-unused string/utils.mbt

Testing

• moon fmt
• moon info
• moon check
• moon test

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https://chatgpt.com/codex/tasks/task_e_68550e1d79c08320a0139902120114fd

[peter-jerry-ye-code-review]

Missing documentation for surrogate pair utilities

Category
Maintainability
Code Snippet
fn is_leading_surrogate(c : Int) -> Bool
fn is_trailing_surrogate(c : Int) -> Bool
fn code_point_of_surrogate_pair(leading : Int, trailing : Int) -> Char
Recommendation
Add documentation comments (///|) explaining each function's purpose, parameters, and return values
Reasoning
These functions handle complex Unicode concepts and should be well-documented to help other developers understand their purpose and proper usage

No bounds checking in code_point_of_surrogate_pair

Category
Correctness
Code Snippet
fn code_point_of_surrogate_pair(leading : Int, trailing : Int) -> Char {
((leading - 0xD800) * 0x400 + trailing - 0xDC00 + 0x10000).unsafe_to_char()
}
Recommendation
Add parameter validation to ensure leading and trailing are valid surrogate values:

fn code_point_of_surrogate_pair(leading : Int, trailing : Int) -> Char {
  if !is_leading_surrogate(leading) || !is_trailing_surrogate(trailing) {
    panic("Invalid surrogate pair")
  }
  ((leading - 0xD800) * 0x400 + trailing - 0xDC00 + 0x10000).unsafe_to_char()
}```
**Reasoning**
The function uses unsafe_to_char() but doesn't validate its inputs, which could lead to undefined behavior if called with invalid surrogate values

</details>
<details>

<summary> Magic numbers in surrogate pair calculation </summary>

**Category**
Maintainability
**Code Snippet**
((leading - 0xD800) * 0x400 + trailing - 0xDC00 + 0x10000).unsafe_to_char()
**Recommendation**
Extract magic numbers into named constants:
```moonbit
let surrogate_offset = 0x10000
let surrogate_shift = 0x400```
**Reasoning**
The surrogate pair calculation uses several magic numbers whose purpose isn't immediately clear. Named constants would make the code more maintainable and self-documenting

</details>

[coveralls]

Pull Request Test Coverage Report for Build 7402

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.886%

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Totals
Change from base Build 7401:	0.0%
Covered Lines:	8505
Relevant Lines:	9462

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💛 - Coveralls