Add generators for variables_get_dynamic and variables_set_dynamic blocks

[rachel-fenichel]

Problem statement

We have blocks in our core library with no generators.

Expected Behavior

All of our core library blocks should have generators.
The dynamic variables_get and variables_set blocks should have the same generators as the non-dynamic versions, since all of our provided languages are dynamic.

Actual Behavior

An assertion fails when trying to generate code for a variables_get_dynamic block.

Steps to Reproduce

1. Create a variable from the second variables category on the playground
2. Click "To JavaScript"
3. Check the console.

Stack Traces

asserts.js:81 Uncaught goog.asserts.AssertionError {message: "Assertion failed: Language "JavaScript" does not k…rate code for block type "variables_get_dynamic".", reportErrorToServer: true, messagePattern: "Assertion failed: Language "%s" does not know how to generate code for block type "%s".", stack: "AssertionError: Assertion failed: Language "JavaSc…blockly/gblockly-rr/tests/playground.html:315:79)"}```

### Operating System and Browser
All