Rule Request: `allowed_prefix_symbols` configuration in `identifier_name`

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

When validates_start_with_lowercase is true, all identifier names must start with a lowercase character.
This will trigger a violation even if the name starts with a symbol, for example and underscore _.

The only way to achieve this now is to disable validates_start_with_lowercase, but it would be nice to have to option to allow certain prefix symbols, eg:

identifier_name:
  allowed_prefix_symbols: ["_"]
  validates_start_with_lowercase: true

The above would trigger a violation for:

var MyVar: String

...but would not trigger a violation for:

private var _myVar: String

While naming properties this way shouldn't be the norm, sometimes it can serve as raw value storage for a computed property of the same name. For example:

private var _myVar: Int?

@objc
public var myVar: Int {
    get { 
         _myVar ?? -1
    }
    set { 
        _myVar = newValue < 0 ? nil : newValue
    }
}

Start by locating SwiftLint’s identifier_name rule and its configuration handling, then trace how validates_start_with_lowercase checks the first character. Add coverage for allowed_prefix_symbols using the shown _myVar and MyVar cases, and confirm the option’s behavior in the rule’s existing tests.