FOMOD validation for CLI

[Al12rs]

• Added static GuidedInstallerValidation class containing two static validation methods, one for a single group, one for an entire step.

• Adds checks before proceeding with steps to make sure each group selection respects the requirements of the group (At least one, at most one, exactly one).

• Fixed Fomod installer creating GamePaths using unsanitized paths.

[codecov]

Codecov Report

Merging #608 (34de8b0) into main (3680f83) will increase coverage by 67.85%.
The diff coverage is 44.23%.

@@            Coverage Diff            @@
##           main     #608       +/-   ##
=========================================
+ Coverage      0   67.85%   +67.85%     
=========================================
  Files         0      600      +600     
  Lines         0    16512    +16512     
  Branches      0     1103     +1103     
=========================================
+ Hits          0    11205    +11205     
- Misses        0     5026     +5026     
- Partials      0      281      +281     

Flag	Coverage Δ
Linux	67.08% <44.23%> (?)
Windows	66.96% <44.23%> (?)
clean_environment_tests	67.85% <44.23%> (?)

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Files Changed	Coverage
src/NexusMods.CLI/CliGuidedInstaller.cs	0.00%
...c/Games/NexusMods.Games.FOMOD/FomodXmlInstaller.cs	80.00%
...GuidedInstaller/GuidedInstallationStepValidator.cs	100.00%