Implement dispose for keyboard-help content

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Related to https://github.com/phetsims/scenery-phet/issues/769 ...

This sim currently does not dispose of keyboard-help content, and currently has dispose implemented like this:

  dispose() {
    assert && assert( false, 'dispose is not supported, exists for the lifetime of the sim' );
    super.dispose();
  }

The sim will therefore fail in the PhET-iO State Wrapper, because it creates and then immediately disposes PhetioCapsules on startup.

Implement dispose for these classes:

• [ ] DiscreteKeyboardHelpContent
• [ ] WaveGameKeyboardHelpContent
• [ ] WavePacketKeyboardHelpContent
• [ ] MeasurementToolsKeyboardHelpSection

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This is not currently a priority because this sim has not had a PhET-iO design, and there are no specific plans for a PhET-iO version or release. So I'm leaving this issue unassigned.

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In the above commit, I implemented dispose for keyboard help content. Using the process and patch in https://github.com/phetsims/scenery-phet/issues/769#issuecomment-1301473507, memory looks good:

• 126MB with 0 KeyboardHelpDialog instances
• 148MB with 10 instances
• 131MB with 100 instances

Closing.