feat(loader): support merge type

[Zekfad]

Implemented according to https://yaml.org/type/merge.html

Supports:

• merge single map
• merge multiple maps
• correctly overrides

Tests are made for all 3 cases and for duplicate merge key as well.

Closes #121

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• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[jonasfj]

I propose that this feature should not be implemented in package:yaml, see https://github.com/dart-lang/yaml/issues/121#issuecomment-2154574354