Extract backing property from property-naming rule to separate rule

[paul-dingemans]

The property-naming also contains logic to check backing properties.

Kotlin Coding Conventions clear states that:

If a class has two properties which are conceptually the same but one is part of a public API and another is an implementation detail, use an underscore as the prefix for the name of the private property:

When a backing property is needed, its name should exactly match that of the real property except prefixed with an underscore. Android Kotlin style guide is less explicit:

When a backing property is needed, its name should exactly match that of the real property except prefixed with an underscore.

Several users have requested to loosen the restrictions regarding the modifier of the base property:

• Backing property requires other value to be part of public API #2345 (comment)
• ktlint 1.1.0 incorrectly reports backing property name error when public-facing variable is internal #2448 (comment)
• ktlint 1.1.0 incorrectly reports backing property name error when public-facing variable is internal #2448 (comment)
• ktlint 1.1.0 incorrectly reports backing property name error when public-facing variable is internal #2448 (comment)
• ktlint 1.1.0 incorrectly reports backing property name error when public-facing variable is internal #2448 (comment)

As Originally posted by @paul-dingemans in #2345 (comment) the solution could be to split the property-naming rule and extract part of the logic to a new rule backing-property. Requirements:

• property-naming rule should ignore properties for which the name starts with _ as this will be handled by backing-property rule
• backing-property rule will be identical to logic which is currently implemented in property-naming in case of intellij_idea or ktlint_official code styles. In case of android_studio code style, the modifier is ignored.