Add coerceInputLiteral()
Removes `valueFromAST()` and adds `coerceInputLiteral()` as an additional export from `coerceInputValue`.
The implementation is almost exactly the same as `valueFromAST()` with a slightly more strict type signature and refactored tests to improve coverage (the file unit test has 100% coverage)
While this does not change any behavior, it could be breaking if you rely directly on the valueFromAST() method. Use `coerceInputLiteral()` as a direct replacement.
