Fix browser-specific manifest generation

Describe the bug
Code for merging _base.json manifest and some browser-specific manifest like chrome.json is not working properly when there is property which value is array defined in both manifests.

Steps to reproduce (REQUIRED)

1. Define one additional permission in the browser-specific manifest, let's say in chrome.json
2. Run yarn dist
3. Check generated manifest for chrome, permission in chrome.json override first permissions from _base.json

Expected behavior
For upper example, generated manifest for chrome should have all permissions from _base.json and additionally permission defined in chrome.json.

In a broader sense, if the same property (array value) is defined in base and the browser-specific manifest resulting manifest should hold all array elements from both _base.json and browser-specific manifest (also should check for duplicates).

Start by running yarn dist and tracing how _base.json is merged with chrome.json during manifest generation. Verify the generated Chrome manifest when both files define an array property; done means retaining all elements from both manifests without duplicates.