Improve Should-BeString to show string diff with arrow marker

[nohwnd]

Fixes #2462

When Should-BeString fails, the error message now shows:

• The difference index
• String lengths (when different)
• Expected and actual strings with expanded special characters (\n, \r, \t, etc.)
• An arrow marker pointing to the first difference

Before:

Expected [string] 'abc', but got [string] 'bde'.

After:

Expected strings to be the same, but they were different.
String lengths are both 3.
Strings differ at index 0.
Expected: 'abc'
But was:  'bde'
          ^

Whitespace differences are visible even without -IgnoreWhitespace:

Expected strings to be the same, but they were different.
Expected length: 8
Actual length:   7
Strings differ at index 3.
Expected: 'abc\r\ndef'
But was:  'abc\ndef'
          ----^