manifest.yml parsing for symlinked extensions

### Describe the feature or problem you’d like to solve

If an extension is installed by a third-party tool (for example nixos/home-manager) and symlinked into place in the `extensions` folder, `gh` will not attempt to see if it has a `manifest.yml` to load data about it.

### Proposed solution

Check the destination for a `manifest.yml` file and load it if found. This will quickly show users more context about their (externally) managed extensions such as version and repo.

### Additional context

Current for externally-linked extensions

```
❯ gh extension list
NAME REPO VERSION
gh copilot
gh kustomize
gh not
gh notify
gh slack
```

Desired

```
❯ bin/gh extension list
NAME REPO VERSION
gh copilot github/gh-copilot v1.1.0
gh kustomize
gh not nobe4/gh-not v0.6.4
gh notify
gh slack
```