Add options to Slider keyboard help, for hiding irrelevant keys

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In some cases, the Shift key may be irrelevant for a slider. And it may therefor be useful to be able to omit the hift key from keyboard help (see SliderControlsKeyboardHelpSection.ts). This use case is currently hypothetical, so labeling this as deferred for now. And we might also consider whether Page Up/Down should similarly be omittable in some cases.

In phetsims/greenhouse-effect#261 (comment), @arouinfar said:

... Since the Layer Model screen does have another slider that supports the shift modifier, the Keyboard Shortcuts dialog content is accurate. However, I wonder what happens in a sim where the sliders do not support smaller/larger steps. Are those sections of the dialog optional @jessegreenberg?

In phetsims/greenhouse-effect#261 (comment), @jessegreenberg replied:

No, there is no option to remove that row currently. We could add that though if you would like. In this case though the shift key still applies to the flux meter sensor so keeping it for this sim may make sense. Let me know if there should be an option for that.

In phetsims/greenhouse-effect#261 (comment), @arouinfar said:

Thanks @jessegreenberg. At this stage, it's a fairly hypothetical case. I think we can wait to add the option until we get to a sim where the smaller and/or larger step modifiers are irrelevant, but I'll leave it up to you to decide.