Add optimised findall(isequal(::Char), ::String) (#54593)
This uses the same approach as the existing findnext and findprev
functions in the same file.
The following benchmark:
```julia
using BenchmarkTools
s = join(rand('A':'z', 10000));
@btime findall(==('c'), s);
```
Gives these results:
* This PR: 3.489 μs
* 1.11-beta1: 31.970 μs
