Enhance Burn's SetVariable so that it can evaluate a formatted string

[rseanhall]

Feature requests

If this issue is a feature request:

• Describe the scenario and benefits that the feature supports.

SetVariable was added in v4, but it only can set a variable to a static value. The analogous SetProperty in MSI evaluates the string so SetVariable should be able to do this, too. This would help when getting a version string from the registry and coercing it to a version to use Burn's version comparisons.

• Describe how you're accomplishing the feature today (if possible).

Custom code in the BA.

• Describe what you'd like the new feature to do.

Change the existing Type attribute so that the source value has a type (SourceType?) and the target variable has a type (keep Type or TargetType?).