feat(stringview): add missing methods for stringview

[Yu-zh]

Part of #1914. This PR only adds methods to View, and does not change other public APIs.

[peter-jerry-ye-code-review]

Several methods contain TODO comments about removing .view() calls

Category
Maintainability
Code Snippet
// todo: remove .view() in new version
String::from_array(chars).view()
Recommendation
Either remove the .view() calls now if possible, or create a tracking issue for this technical debt. Add justification in comments why this needs to be done later.
Reasoning
TODO comments tend to be forgotten. Having a clear plan for addressing these in future helps maintain code quality and provides context for other developers.

iter2() function has complex loop control flow that could be error-prone

Category
Correctness
Code Snippet
continue index + 2, n + 1
Recommendation
Consider splitting the surrogate pair handling into a separate helper function to make the logic more clear and testable:

fn handle_surrogate_pair(view: View, index: Int, n: Int) -> (Int, Int, Char)? {...}```
**Reasoning**
The current implementation mixes multiple concerns: iteration, surrogate pair detection, and yield control. Separating these would make the code more maintainable and easier to verify for correctness.

</details>
<details>

<summary> Documentation is missing for iter2() method </summary>

**Category**
Maintainability
**Code Snippet**
///|
pub fn View::iter2(self : View) -> Iter2[Int, Char]
**Recommendation**
Add documentation explaining the purpose of iter2(), especially how it differs from iter(). Include examples if possible.
**Reasoning**
All other public methods have documentation explaining their purpose. Consistent documentation helps users understand the API better.

</details>

[coveralls]

Pull Request Test Coverage Report for Build 6197

Details

• 14 of 18 (77.78%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 91.56%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
string/view.mbt	14	18	77.78%

Totals
Change from base Build 6196:	-0.02%
Covered Lines:	5641
Relevant Lines:	6161

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💛 - Coveralls