Rename Lexer.isAtStartOfPostfixExprSuffix back

Author: natikgadzhi

Sources/SwiftParser/Expressions.swift

@@ -874,7 +874,7 @@ extension Parser {
             }
           } while lookahead.at(.poundIf) && lookahead.hasProgressed(&loopProgress)
 
-          guard lookahead.atStartOfPostfixExprSuffix() else {
+          guard lookahead.isAtStartOfPostfixExprSuffix() else {
             break
           }
         }
@@ -2487,7 +2487,7 @@ extension Parser.Lookahead {
 extension Parser.Lookahead {
   // Helper function to see if we can parse member reference like suffixes
   // inside '#if'.
-  fileprivate mutating func atStartOfPostfixExprSuffix() -> Bool {
+  fileprivate mutating func isAtStartOfPostfixExprSuffix() -> Bool {
     guard self.at(.period) else {
       return false
     }