enforce return type in all functions

[wkozyra95]

Checklist

• I've added an entry to CHANGELOG.md if necessary.
• I've tagged the changelog entry with [EAS BUILD API] if it's a part of a breaking change to EAS Build API (only possible when updating @expo/eas-build-job package).

Why

enforce explicit definition of return type

Test Plan

tested branch:list command because it was only change that was not purely affecting types

[github-actions]

Size Change: -2.08 kB (0%)

Total Size: 27.1 MB

Filename	Size	Change
./packages/eas-cli/dist/eas-linux-x64.tar.gz	27.1 MB	-2.08 kB (0%)

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