perf: Make ViewModelBase's internal errors dictionary lazy initialized

[jeanplevesque]

GitHub Issue: #

Proposed Changes

• Bug fix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes, no api changes)
• Build or CI related changes
• Documentation content changes
• Other, please describe: Performance optimization

What is the current behavior?

• The internal _errors dictionary for the INotifyDataErrorInfo aspect of ViewModelBase is always instantiated, even if the INotifyDataErrorInfo features aren't used for a ViewModel.
• The _errors dictionary is a ConcurrentDictionary.

Method	Mean	Allocated
CreateViewModel	2.057 μs	1760 B
CreateViewModel_WithExplicitName	2.077 μs	1760 B

What is the new behavior?

• The _errors field is now a Dictionary. We weren't using any features specific to ConcurrentDictionary.
• The _errors field is now lazy-initialized. It stays null as long a we don't call SetErrors.

Method	Mean	Allocated
CreateViewModel	1.280 μs	936 B
CreateViewModel_WithExplicitName	1.193 μs	936 B

Impact on version

• Major (Public API was modified.)
  • Public constructs (class, struct, delegate, enum, etc.) were removed or renamed.
  • Public members were removed or renamed.
  • Public method signatures were changed or renamed.
• Minor (Public API was extended.)
  • Public constructs, members, or overloads were added.
• Patch (Public API was unchanged.)
  • A bug in behavior was fixed.
  • Documentation was changed.
• None (The library is unchanged.)
  • Only code under the build folder was changed.
  • Only code under the .github folder was changed.

Checklist

Please check that your PR fulfills the following requirements:

• Documentation has been added/updated.
• Automated Unit / Integration tests for the changes have been added/updated.
• Updated BREAKING_CHANGES.md (if you introduced a breaking change).
• Your conventional commits are aligned with the Impact on version section.

Other information