Make masked and expand options mutually exclusive in variables UI

Proposal

A masked variable cannot contain the $ character. That character is needed to reference a variable that would get expanded with Expand variable reference enabled. This means that the two settings are functionally incompatible: You can create a masked and expanded variable – but you can't put a value into it that would actually get expanded.

We should consider disabling the expansion option when the masked option is selected in the UI for creating/editing variables to avoid confusion.

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This came up in a docs update about masking requirements: !203480 (merged)