Fix: allow build with no selected build preset

[bailwillharr]

This pull requests fixes the behaviour I mentioned in Issue #333 (CMake build presets are required when using configure presets). It allows the user to proceed with building if a CMake configure preset has been selected but no build presets are available.

This change was implemented by creating a sentinel object for the BuildPreset class which represents the choice of no build preset. This choice cannot be specified with build_preset = nil as that represents the case where a build preset is yet to be selected.

Code can check for this 'no build preset wanted' state by calling build_preset.is_none which returns true in that case.

A few if statements in lua/cmake-tools/init.lua are modified to ensure that build_preset is not used when set to BuildPreset.None.

In addition, a minor typo causing undersired behaviour was fixed on line 33 (previously line 21) of lua/cmake-tools/build_preset.lua.

[lceWolf]

Nice finding and I very much appreciate this MR!
While your changes do work, I think the handling of the "None" build-preset itself could be improved.
I don't like the special handling for a "None" choice and checking for that hardcoded string.

Hope you don't feel bad about this, but I've got another idea for a solution. I'd love hearing your feedback on #335

[lceWolf]

That does not actually return a new instance. It returns the same table for each invocation. Modifications of that table would be shared across every "instance" you created that way. To properly create a new instance you should use
return setmetatable({}, <NonePreset-Table>)

[lceWolf]

I think this is not quite the effect you are trying to achieve. This is not an instance. It is something like a subclass within the BuildPreset class which re-uses the methods and fields of the BuildPreset class, yet some methods are overwritten/hidden (get_build_target and get_build_type)

See my comment on the createEmptyBuildPreset method on how to create individual instances of a None Preset

[lceWolf]

Closed in favor of #335

[bailwillharr]

Thanks a lot for this fix man. Tbh my experience with Lua is pretty limited so I'm glad someone who knows what they're doing stepped up :).