IsAnagram bug (per element counts)

### Describe the bug
The `IsAnagram` function is not checking per-element counts. It is only checking if they have the same elements, but not if the count of each element matches.

A more appropriate name for the current logic is something like `ContainsNoDifferingElements` or `IntersectsMatch` rather than `IsAnagram`. I would recommend changing the name or the logic of the method.

> Note: If you aren't going to check per-element counts, then you should also get rid of this check in `IsAnagrams`:
> ```cs
> if (source.Length != other.Length)
> return false;
> ```
> because length doesn't matter if you don't also check per-element counts.

### To Reproduce
Add the following case to the `IsAnagram` unit tests:
```cs
string aab = "aab";
string abb = "abb";
Assert.False(Permutations.IsAnargram(aab, abb));
```

### Expected behavior
Spans of the same length and elements but different per-element counts should not be considered re-orders/anagrams of each other.

### Environment:
_master branch_

### Additional context
I have written my own version of this algorithm in C# _(that fixes this issue)_ if interested here...
> Source Code: https://github.com/ZacharyPatten/Towel/blob/d2660e208ad3a44ab22f192834760c5b93dc82ac/Sources/Towel/Statics-SequenceAnalysis.cs#L1321
> Examples: https://github.com/ZacharyPatten/Towel/blob/d2660e208ad3a44ab22f192834760c5b93dc82ac/Examples/BasicsAndExtensions/Program.cs#L406
> Testing: https://github.com/ZacharyPatten/Towel/blob/d2660e208ad3a44ab22f192834760c5b93dc82ac/Tools/Towel_Testing/Statics.cs#L2086
> _Note: `MapHashLinked` is my version of a `Dictionary` if you look at the source code._

The issue is in the Permutations.IsAnagram method. Look at the unit tests to understand the current behavior. The bug is that it doesn't check per-element counts. The fix involves updating the algorithm to count occurrences of each character. The provided external link shows a corrected implementation. Add the failing test case first, then modify the method to pass it.

Written by the indexing model from the issue text.