[WIP] Add lints to enforce type safety

[Copilot]

Plan: Add Type-Safety Lints to fd_lints

Based on the existing lints that focus on type-safety (avoid_as, avoid_non_null_assertion, and the unmodifiable view mutations), I will add the following complementary type-safety lints:

• Research and identify relevant type-safety lints to add
• Implement avoid_dynamic_calls - Warn when calling methods/properties on dynamic types
• Implement prefer_is_empty - Prefer .isEmpty over .length == 0 for better null-safety
• Implement avoid_returning_null - Avoid explicitly returning null (prefer nullable return types)
• Create documentation for each new lint in the doc/ directory
• Update lib/main.dart to register the new lints
• Update README.md to include the new lints in the table
• Update CHANGELOG.md with the new additions
• Test the lints by running dart analyze on the example
• Verify all changes work correctly

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