🐛 fix: merging of two similarly keyed mappings

[StummeJ]

Overview

This sets out to fix issue #10. We found that merging two sources with similar mappings would cause the destruction of the mapping from the previously processed source. The use case we found this in was having a config with some secret values, then populating just the secret values from a secure store.

Specifics

• Add deepmerge package to handle merging of two mappings properly
• Add some additional typing for added type safety
• Fix a test that would fail due to vscode using TEST_ environment variables for it's test runner

[RobertoPrevato]

Hi @StummeJ
I apologize for replying so late, I appreciate this very much and I am willing to merge and release a new package with this feature.