Update [GeneratedRegex] analyzer/fixer for partial properties

[stephentoub]

The analyzer will currently flag fields like:

private static readonly Regex s_regex = new Regex(...);

and the fixer will propose generating a new method instead. Now that partial properties are a thing and are supported by the generator, the fixer can instead just change the field to be a property (and a user can subsequently rename it if desired), e.g.

[GeneratedRegex(...)]
private static partial Regex s_regex { get; }

For cases where the analyzer flags a property, it's even easier: it can just make the property partial, remove the initialization, and add the attribute.

And for cases where something new still needs to be generated, the fixer could offer the choice of a method or property.

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