config to not pass '-j' to cmake

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### Brief Issue Summary

Cmake-tools always adds '-j' to the cmake-invocation. So its not possible to use other methods that cmake supoorts for setting the parallelism value, like a environmentVariables in cmake-tools-kits.json.

### CMake Tools Diagnostics

_No response_

### Debug Log

_No response_

### Additional Information

I have a project set up in a way so that the number of build jobs is supplied by another tool.
But I found no way to make cmake-tools not add any "-j X" flag to the call of cmake.

It seemed that #2187 intended to add this behavior when cmake.parallelJobs is 1, but now currently, 1 forces '-j 1' on the command line.

In theory, cmake.parallelJobs=null has the right behavior, but the json config validator of vscode does not like this, so I have to manually edit the config.json.

I would suggest to allow null in the config (which probably means adding it here: https://github.com/microsoft/vscode-cmake-tools/blob/c66d1eab6248f5fb82e128cc10ecbab95b95285c/src/config.ts#L473),
and to add a clearer explanation of what 1 vs undefined to to the setting description.

Start in src/config.ts around the cmake.parallelJobs setting and review how its configuration value is validated and described. Confirm that null is accepted and that the setting text clearly distinguishes 1 from undefined or null; done means VS Code no longer rejects the configuration and users can omit the -j argument.

Written by the indexing model from the issue text.