Environment section in defaults completely overridden by profile-specific one

### Windows Terminal version

1.19.3172.0

### Windows build number

10.0.22631.2861

### Other Software

_No response_

### Steps to reproduce

1. Create `environment` section inside `defaults` settings and add variable A
`"environment": { "A": "AValue" }`
1. Create `environment` section inside a profile add variable B
`"environment": { "B": "BValue" }`

### Expected Behavior

Both A and B are expected to be defined

### Actual Behavior

Only B is defined

The reason is probably because this section is treated just like any other setting, which when overridden by profile-specific setting it takes nothing from the default section, but this isn't the expected behavior for environment variables, as long as they have different names

Start by tracing how the defaults and profile-specific settings merge, focusing on the environment section described in the reproduction. Confirm the current behavior with the A and B configuration examples, then verify that both variables remain defined when their names differ.

Written by the indexing model from the issue text.