Change checkSourceLengthRule to use Span.IsWhiteSpace instead of Stri…

[Numpsy]

…ng.Trim

String.Trim can allocate a new string on each call, whereas Span.IsWhiteSpace() shouldn't allocate at all.

Just a thought when looking at some Span related things.
If I run the existing benchmark app using the rule set which is enabled during the SelfCheck part of the CI build then I get this with the current code

| Method         | Mean    | Error    | StdDev   | Gen0       | Gen1       | Gen2      | Allocated |
|--------------- |--------:|---------:|---------:|-----------:|-----------:|----------:|----------:|
| LintParsedFile | 1.442 s | 0.0155 s | 0.0145 s | 43000.0000 | 13000.0000 | 3000.0000 | 726.17 MB |

And this afterwards

| Method         | Mean    | Error    | StdDev   | Gen0       | Gen1       | Gen2      | Allocated |
|--------------- |--------:|---------:|---------:|-----------:|-----------:|----------:|----------:|
| LintParsedFile | 1.454 s | 0.0094 s | 0.0073 s | 42000.0000 | 13000.0000 | 3000.0000 | 722.76 MB |

A small change improvement given the total allocations, but also a small change so I thought it'd still be useful.