refactor(plugin-webpack): move webpack config generation code to its own file

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Splits out the Webpack config generation code to its own file, and adds basic test coverage.

[codecov]

Codecov Report

Merging #1401 into master will decrease coverage by 26.66%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1401       +/-   ##
===========================================
- Coverage    68.5%   41.84%   -26.67%     
===========================================
  Files          72        4       -68     
  Lines        2099      184     -1915     
  Branches      389       37      -352     
===========================================
- Hits         1438       77     -1361     
+ Misses        507       95      -412     
+ Partials      154       12      -142

Impacted Files	Coverage Δ
packages/api/core/src/api/import.ts	26.73% <0%> (-32.1%)	⬇️
packages/maker/snap/src/MakerSnap.ts
...es/api/core/src/api/init-scripts/init-directory.ts
packages/api/core/src/util/index.ts
packages/api/core/src/util/publish-state.ts
packages/utils/async-ora/src/index.ts
packages/maker/wix/src/util/author-name.ts
packages/maker/rpm/src/MakerRpm.ts
packages/utils/test-utils/src/index.ts
...ypescript-webpack/src/TypeScriptWebpackTemplate.ts
... and 58 more

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