Add a custom key code that means "control on windows and command on mac"

[maribethb]

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Problem

For keyboard shortcuts that use "control" on windows and "command" on mac, the author of the shortcut has to manually register both keys (and blockly calls the "command" key "meta" which is potentially confusing as windows also has a separate "meta" key)

Request

Add a custom key code to the list of allowable modifier keys. If this key code is registered to a shortcut, blockly will automatically use the appropriate platform-specific key.

Name tbd: CTRL_CMD ?

This is inspired by the webdriver custom keycode that has this behavior.

Alternatives considered

No response

Additional context

No response