credentials: file authoring/editing language support

As a user, when editing `~/.aws/*` credentials files such as `~/.aws/credentials` or `~/.aws/config`, I want syntax support for AWS credentials files:

- MVP
- Auto-completion
- Syntax highlight
- Syntax check / errors
- Future (LSP based?)
- File outline
- Go to Definition
- Find References
- Smart rename

## Notes

- Sections can have subkeys! ([ref](https://github.com/aws/aws-sdk/pull/230))
- Introduced in 2014 for s3: https://docs.aws.amazon.com/cli/latest/topic/s3-config.html
- Example:
```
[profile development]
aws_access_key_id=foo
aws_secret_access_key=bar
s3 =
max_concurrent_requests = 20
max_queue_size = 10000
multipart_threshold = 64MB
```

Start with the ~/.aws/credentials and ~/.aws/config entry points and review the linked AWS CLI configuration guidance, paying attention to sections with nested subkeys. Done means the MVP provides auto-completion, syntax highlighting, and syntax errors for these files; the listed LSP features are future work.

Written by the indexing model from the issue text.