add service interface for typescript

[encendre]

Attempt to solve #450

I modified getMethodTypings in the typescript generator so that it returns the interface as well.
Another solution would be to export a separate function that generates the interface.

[codecov]

Codecov Report

Merging #451 (76ca637) into master (e168e2b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #451   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          167       172    +5     
  Branches        23        25    +2     
=========================================
+ Hits           167       172    +5     

Impacted Files	Coverage Δ
src/typescript.ts	100.00% <100.00%> (ø)

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[BelfordZ]

awesome work @encendre

Ill check this out asap!