Add missing type declaration for getCombinedSourcemap

[Anidetrix]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

[codecov]

Codecov Report

Merging #3431 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3431   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files         171      171           
  Lines        5831     5831           
  Branches     1722     1722           
=======================================
  Hits         5540     5540           
  Misses        157      157           
  Partials      134      134           

Impacted Files	Coverage Δ
src/utils/transform.ts	98.43% <ø> (ø)

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[lukastaegert]

Thanks for spotting this. There was one issue with your implementation, namely that getCombinedSourcemap is only available in the transform hook but otherwise undefined. This was also spotted by our typings test (npm run test:typescript). I was able to fix this on your branch, so now it is good to be merged.