Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones. For example:
#include <cm/string_view>
can be used safely for CMake development in place of the `<string_view>`
standard header.
Fixes: #19491
This replaces invocations of
- `cmSystemTools::IsInternallyOn` with `cmIsInternallyOn`
- `cmSystemTools::IsNOTFOUND` with `cmIsNOTFOUND`
- `cmSystemTools::IsOn` with `cmIsOn`
- `cmSystemTools::IsOff` with `cmIsOff`
Ext and External were used inconsistently in the code and the
docs. This change converts all uses of Ext to External, including
within variable names used by the generator.
