tstypes: fix typings for 'isSpecifiedDirective'/'isSpecifiedScalarType'

[IvanGoncharov]

Fixes #2153

[IvanGoncharov]

Since we still discussing a long-term solution in #2194 vs #2196 for 14.x.x branch it would be best to just revert to boolean since it was what we had in DefinetlyTyped anyway:

graphql-js/tstypes/type/scalars.d.ts

Line 11 in d1391ee

export function isSpecifiedScalarType(type: GraphQLScalarType): boolean;