feat: add file input flag (#12)

## Description

The lambda CLI previously only supported inline string expressions and stdin input.
This PR adds support for reading lambda expressions from files using the `-f` flag.
This makes it easier to work with larger programs stored in files.

Changes:
- Added `FileSource` type to `internal/config/source.go` for reading from file paths.
- Added `-f` flag to CLI argument parser with validation to prevent conflicting inputs.
- Updated Makefile targets (`run`, `profile`, `explain`) to use `-f` flag instead of stdin redirection.

### Decisions

The `-f` flag takes precedence over positional arguments.
If both are specified, an error is returned to avoid ambiguity.

## Benefits

- More intuitive workflow for file-based lambda programs.
- Cleaner Makefile targets without stdin redirection.
- Consistent with common CLI conventions (e.g., `grep -f`, `awk -f`).

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [ ] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #12
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

internal/config/source.go

@@ -27,3 +27,15 @@ func (s StdinSource) Extract() (string, error) {
 
 	return string(data), nil
 }
+
+// A source reading from a file.
+type FileSource struct{ Path string }
+
+func (s FileSource) Extract() (string, error) {
+	data, err := os.ReadFile(s.Path)
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}