Introduce support for escaping the escape character for property placeholders

Up until spring-boot 3.3.8, an escaped backslash followed by a placeholder like:

prop1=value1  
prop2=value2\\${prop1}  

Resulted in prop2 resolving to:

value2\value1

Starting with spring-boot 3.4.0, the result is:

value2value1

Doubling the backslashes like this:

prop1=value1  
prop2=value2\\\\${prop1}  

Results in:

value2${prop1}

This is due to the escaping logic introduced in org.springframework.util.PlaceholderParser which does two passes of PlaceholderParser.SimplePlaceholderPart.resolveRecursively(PartResolutionContext, String), each calling PlaceholderParser.parse(String, boolean) in which the escaping logic is implemented.

This last method does not handle escaping the escape character, causing the issue.

Start with org.springframework.util.PlaceholderParser and its SimplePlaceholderPart.resolveRecursively(PartResolutionContext, String) and parse(String, boolean) methods. Reproduce the property-placeholder examples from the issue and trace both parsing passes. Done means an escaped escape character preserves the expected backslash while placeholders still resolve correctly.