improve new leaf type `parseConstLiteral()` method (#4218)
the new method was introduced in #3812 with upgrade path improved in
#4209.
This PR completes polishes this work a bit:
1. The method is renamed to `coerceInputLiteral()` following the naming
convention suggested in #2357, with the other methods to be renamed in
later PRs.
2. The type of the method `GraphQLScalarInputLiteralCoercer` is
exported, which was not done in the initial work. The old
`GraphQLScalarLiteralParser` type is deprecated.
