chore: use golangci-lint for linting (#3)

Use the [GitHub
Action](https://github.com/golangci/golangci-lint-action) provided by
[`golangci-lint`](https://golangci-lint.run/) to run linting for this
project. `golangci-lint` is one of the most popular linters for Go out
there with great integration/support for VS Code too.

feat: set up basic tests & CI pipeline (#1)

# Why

The current library is very rudimentary, without any unit test or a CI
pipeline. Let's give it a glow up 🪄

# What

- Added basic unit tests for AASA validation
  - Current test coverage (after this change): 13.5%
- Fixed some small bugs discovered as part of adding the tests
- Set up GitHub Actions pipeline for building & running tests
- Added ability to validate a local AASA file so that we can use it in
CI pipeline to validate the file _pre-deployment_
  - For example: `yurl aasa validate --file=./path/to/local/file`

```sh
> yurl aasa validate --help
Validate your link or local file against Apple's requirements

Usage:
  yurl aasa validate [<URL>] [flags]

Flags:
  -f, --file string   path to local file, will ignore the URL
  -h, --help          help for validate
```

Example of running the tool locally (without building the binary):

```sh
> go run cli/main.go aasa validate --file=./yurllib/testdata/apple-app-site-association
JSON Validation:                  Pass
JSON Schema:                      Pass

{
    "applinks": {
        "details": [
...
```