Make `nasm.props` consistent with the `install_script.bat` script

- typo fix in comment
- At least on some Win10 machines, attempting to execute a *non-existent* `"%NASMPATH%\nasm.exe"` will cause the subsequent `%ERRORLEVEL% equ 0` check to **pass** inadvertently, hence we MUST first check whether the indicated nasm.exe exists, truly, before running this version test/check in the installer batch script.
- At least on Win10, when NASM had already been previously installed using the standard Windows NASM installer application, the path to the nasm.exe WILL contain spaces (`C:/Program Files...`) and we must make sure our installer batch script correctly deals with this scenario, requiring paths to be surrounded by `"` double quotes.
- Expanded the error/progress messages in the installer batch script a bit to make them more useful and legible when the `NASMPATH` environment variable has not been set up or is (probably erroneously) *empty*: now at least one of the installer progress messages allows the user to diagnose this scenario.

- be more obvious about the listed paths in the manual-installation section to be specific to the Visual Studio Community Edition. Enterprise et al store their stuff in different paths, named accordingly.
- the manual-installation section lacked the example path for the MSVC2022 Community Edition. Added.

Make `nasm.props` consistent with the `install_script.bat` script

README fixes/tweaks for MSVC2022 / Community Edition

further fixes for in installer batch script when NASM or its installer files are already present on the machine.

updated install script to include support for Visual Studio 2026

Parallel NASM Builds and various fixes