Add JSON configuration support with environment variable interpolation

[Copilot]

This PR adds support for JSON-based configuration files in addition to the existing environment variable approach, addressing the need for more flexible configuration management.

Features Added

JSON Configuration Files

• Supports configuration files in multiple locations:
  • ./tailportal.json (current directory)
  • ./.tailportal.json (current directory, hidden)
  • ~/.tailportal.json (home directory)

Environment Variable Interpolation

• JSON configuration supports $ENV_VAR_NAME syntax for environment variable interpolation
• Example configuration:

{
  "tsAuthKey": "$TS_AUTH_KEY",
  "pulumiPassphrase": "$PULUMI_CONFIG_PASSPHRASE",
  "vultrApiKey": "$VULTR_API_KEY",
  "googleProject": "my-project",
  "googleCredentials": "$GOOGLE_CREDENTIALS"
}

Backward Compatibility

• Maintains full backward compatibility with existing environment variable approach
• JSON configuration takes precedence over environment variables when both are present
• Invalid JSON files gracefully fall back to environment variables with warning messages

Implementation Details

• Added src/config.ts module with centralized configuration loading logic
• Refactored index.ts to eliminate code duplication across all command actions
• Reduced lines of code while adding functionality (54 lines removed, 6 lines added in main file)
• All existing tests continue to pass without modification

Usage Examples

Environment Variables (existing approach):

export TS_AUTH_KEY=your_key
export PULUMI_CONFIG_PASSPHRASE=your_passphrase
tailportal create vultr ewr

JSON Configuration:

echo '{"tsAuthKey": "your_key", "pulumiPassphrase": "your_passphrase"}' > tailportal.json
tailportal create vultr ewr

Mixed approach with interpolation:

export VULTR_API_KEY=secret_key
echo '{"tsAuthKey": "direct_value", "vultrApiKey": "$VULTR_API_KEY"}' > tailportal.json
tailportal create vultr ewr

Fixes #12.

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[Yukaii]

@copilot Update the config under XDG_CONFIG_HOME/tailportal/config.json or ~/.config/tailportal/config.json then ~/.tailportal.json

[Copilot]

@copilot Update the config under XDG_CONFIG_HOME/tailportal/config.json or ~/.config/tailportal/config.json then ~/.tailportal.json

Updated configuration paths to follow XDG Base Directory standards. The config now searches in order:

1. $XDG_CONFIG_HOME/tailportal/config.json (if set)
2. ~/.config/tailportal/config.json (fallback)
3. ~/.tailportal.json (backward compatibility)

Documentation has been updated accordingly. Changes in commit 1f047ed.

[Yukaii]

@copilot COuld you also add some tests for it

[Copilot]

@copilot COuld you also add some tests for it

Added comprehensive unit tests for the configuration functionality. The test suite includes 14 test cases covering:

• Environment variable configuration loading and validation
• JSON configuration files from all supported paths (XDG, ~/.config, ~/.tailportal.json)
• Environment variable interpolation in JSON configs
• Configuration precedence and merging
• Error handling for invalid JSON files

All tests pass successfully. Commit 78c506e.