Update OZ solhint custom rules [WIP]

[gonzaotc]

1. Removes interface-names custom rule which is now covered by solhint's already included interface-starts-with-i
2. Adds interface-only-external-functions rule.
3. Prevents leading underscore in constants and immutables, which was previously only prevented in private constants
4. Prevents leading underscore in public and external functions.
5. Prevents external virtual functions.

Note that 2, 3, and 4 require breaking changes that can be potential candidates for 6.0

Ideally I would like to add another rule to verify that Interfaces can only import other interfaces

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[Amxx]

For reference, there is already this PR that deals with linter rules: #5497

[Amxx]

Ideally I would like to add another rule to verify that Interfaces can only import other interfaces

AFAIK, this is already something solidity enforces. We don't need the linter to check taht if the compiler rejects it anyway.

Edit: that is true of "inheritance", importing is a different thing.

[Amxx]

How efficient/explicit is /^_/.test(node.name) compared to node.name.startsWith('_')