Fix doskey macros for inputs with >1 consecutive whitespace (#17129)

Initially the PR restored the original v1 conhost code for
`MatchAndCopyAlias` which fixed the linked issue.
Afterwards, I've taken the liberty to rewrite the code to use modern
constructs again, primarily `string_view`. Additionally, the v1 code
first counted the number of arguments and then iterated through it
again to assemble them. This new code does both things at once.

Closes #15736

## Validation Steps Performed
The unit tests have been extended to cover multiple consecutive
spaces. All tests pass.