Rewrite core.relax_integer_vars transformation

[emma58]

Fixes # .

Summary/Motivation:

This rewrites the core.relax_integer_vars transformation, modernizing it to our more recent transformation conventions. In particular, it:

• Deprecates the undo argument in favor of implementing reverse according to the design we introduced in Adding (reversible) gdp.transform_current_disjunctive_logic transformation #2809.
• Adds a targets argument
• Optionally finds Vars from active expressions rather than using component_data_objects(Var)

Changes proposed in this PR:

• Rewrites core.relax_integer_vars transformation
• Adds tests!
• Adds VarCollector enum to distinguish getting Vars from active expressions vs. using component_data_objects(Var). Naming suggestions are welcome here...

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[emma58]

This is ready for review again, @mrmundt and @jsiirola