Distinguish between "hard" and "soft" literal types

Explore the idea of internally tracking "hard" and "soft" literal types internally and changing inference and constraint solving based on this distinction.

There have been multiple requests for pyright to retain literal types selectively under certain conditions.

Mypy appears to implement a variant of this idea, visible to the user as `Literal[1]?` versus `Literal[1]`.

The issue names no files, tests, or concrete entry points. Start by examining Pyright's literal-type inference and constraint-solving implementation, then compare the proposed hard/soft distinction with mypy's Literal behavior; the work is not complete until the intended inference rules and acceptance tests are defined.

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