feat: Add configurable table styles for coverage output

[Copilot]

This PR adds the ability to configure table styles for text table output, addressing a key limitation mentioned in the "Not Supported" section of the README.

What's New

Users can now customize the appearance of table output using five different styles from the jedib0t/go-pretty library:

• default - ASCII characters (+, -, |) - compatible with all terminals
• light - Unicode light box drawing characters (current default)
• bold - Unicode bold box drawing characters
• rounded - Unicode rounded corner characters
• double - Unicode double line characters

Usage

Via CLI flag:

go-covercheck --table-style=bold coverage.out
go-covercheck --table-style=rounded --show-history

Via configuration file:

# .go-covercheck.yml
tableStyle: bold
statementThreshold: 70
blockThreshold: 50

Implementation Details

• Added tableStyle field to Config struct with full YAML support
• Added --table-style CLI flag with validation
• Updated both coverage tables and history tables to respect the configuration
• Table style is correctly ignored for non-table output formats (JSON, YAML, etc.)
• Added comprehensive validation and error handling
• Updated sample configuration file and documentation

Visual Examples

Bold style:

┏━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━┓
┃       FILE       ┃    STATEMENTS   ┃   STATEMENT %   ┃
┣━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━┫
┃ main.go          ┃            5/10 ┃            50.0 ┃
┗━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━┛

Rounded style:

╭──────────────────┬─────────────────┬─────────────────╮
│       FILE       │    STATEMENTS   │   STATEMENT %   │
├──────────────────┼─────────────────┼─────────────────┤
│ main.go          │            5/10 │            50.0 │
╰──────────────────┴─────────────────┴─────────────────╯

All existing functionality remains unchanged, and the feature gracefully degrades for terminals that don't support Unicode characters.

Fixes #42.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.