Rename faker.fake() to faker.generate(), add IFaker interface.

[ksaaskil]

• It feels silly to write faker.fake(req), generate() seems easier to understand
• Add IFaker interface, I think for example unmock-fetch could be changed to also consume an IFaker instead of accepting CreateResponse.

[codecov-io]

Codecov Report

Merging #362 into dev will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev    #362      +/-   ##
=========================================
+ Coverage   80.39%   80.4%   +<.01%     
=========================================
  Files          54      54              
  Lines        2413    2414       +1     
  Branches      583     583              
=========================================
+ Hits         1940    1941       +1     
  Misses        473     473

Impacted Files	Coverage Δ
packages/unmock-core/src/faker/index.ts	80.76% <100%> (ø)	:arrow_up:
packages/unmock-core/src/backend/index.ts	89.28% <100%> (+0.19%)	:arrow_up:

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