fix: improve solhint custom rules and remove TODOs

[crStiv]

This commit improves the solhint custom rules by:

1. Expanding visibility check for constant variables to include internal constants
2. Re-enabling and fixing the no-external-virtual rule with:
   • Skip checks for constructors and interfaces
   • Improved error message with actionable suggestion
   • Fixed comparison operator

Both TODO comments have been addressed while maintaining the codebase's
coding standards and improving the overall code quality checks.

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[Amxx]

NOTE: prettier rules say that receive and fallback function MUST be external. This creates a conflicts if they are virtual. This leaves us tree options:

• remove virtual from receive/fallback: bit no for me!
• change the prettier config
• (what I did here) skip the rule for receive and fallback functions.

[crStiv]

@Amxx what are your thoughts on this PR altogether? I've made some changes to get rid of red failed checks, but they still fail