refactor(csv): remove `runeCount()` function

[timreichen]

• replaces runeCount() with length of strings

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.81%. Comparing base (73f236d) to head (12b53eb).

Files	Patch %	Lines
csv/_io.ts	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5298 +/- ## ========================================== - Coverage 95.81% 95.81% -0.01% ========================================== Files 457 457 Lines 37907 37895 -12 Branches 5568 5568 ========================================== - Hits 36321 36309 -12 Misses 1546 1546 Partials 40 40 ```

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[kt3k]

This makes the count of characters broken for surrogate pair characters

> "🐱".length
2
> Array.from("🐱").length
1

[timreichen]

This makes the count of characters broken for surrogate pair characters

> "🐱".length
2
> Array.from("🐱").length
1

Ah damn, sorry for the oversight. I think we should add a test for that.