define resolver type

[Quramy]

I want TypeScript support to know what methods/props the context cache has.

[codecov-io]

Codecov Report

Merging #340 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #340   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files           2      2           
  Lines          80     80           
  Branches       22     22           
=====================================
  Hits           76     76           
  Misses          1      1           
  Partials        3      3

Impacted Files	Coverage Δ
packages/apollo-link-state/src/index.ts	93.65% <ø> (ø)	⬆️

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