feat(compute/serve): implement --watch-dir flag

[Integralist]

Fixes #646

Context

The --watch flag (for the compute serve command) indicates to the Fastly CLI that all files within the Compute@Edge project directory need to be watched for changes.

The Fastly CLI will respect a .ignore or .gitignore file within the project directory (it also respects the user's global git ignore configuration as well).

New Feature

I've added a new --watch-dir flag which informs the Fastly CLI which directory it should start watching files from (defaults to the current directory, which is expected to be the associated Compute@Edge project directory).

Example

I've created a Compute@Edge project nested within another directory:

.
├── compute
│   ├── .gitignore
│   ├── Cargo.toml
│   ├── README.md
│   ├── fastly.toml
│   ├── rust-toolchain.toml
│   ├── src
└── foo

NOTE: foo (in the above tree output) is just a text file

The Fastly CLI must be run inside the compute directory as that is the Compute@Edge project, but now a user can provide the --watch-dir flag to indicate that changes to files outside of the directory need to be watched:

fastly compute serve --watch --watch-dir ../ --verbose

The above example uses a relative path from the compute directory up to the parent directory, but an absolute path works as well:

fastly compute serve --watch --watch-dir /Users/integralist/Code/test-projects/testing-fastly-cli/ --verbose

CAVEAT: The Fastly CLI only considers one set of ignore files. For example, with the above command executed, although I have .gitignore defined inside the compute directory, those patterns will not be used. Once the -watch-dir flag is set, it will look inside its specified directory for any ignore configuration.

[kailan]

Looks good!