Refactor the plugin to use the lexical variable mixin from App Inventor

Remove the mixin defined methods from each block.
Add the methods required by the mixin to each block.
Mix the mixin into each block. Test the result.
For each block.foo(...) method defined in the mixin create a new foo(block, ...) function.
Change the code in field_lexical_variable.js (and any other code) that calls a block.foo(...) method defined in the mixin to call the new foo(block, ...) function instead. Test the result.
Remove the mixin

Note that once this is done, we can then do following to App Inventor (similar to the above):

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