Rename Parser.Lookahead functions from is* to at*

Author: natikgadzhi

Sources/SwiftParser/Expressions.swift

@@ -1972,7 +1972,7 @@ extension Parser.Lookahead {
 
     // If this is the start of a get/set accessor, then it isn't a trailing
     // closure.
-    guard !self.withLookahead({ $0.isStartOfGetSetAccessor() }) else {
+    guard !self.withLookahead({ $0.atStartOfGetSetAccessor() }) else {
       return false
     }
 

Sources/SwiftParser/Lookahead.swift

@@ -246,7 +246,7 @@ extension Parser.Lookahead {
 // MARK: Lookahead
 
 extension Parser.Lookahead {
-  mutating func isStartOfGetSetAccessor() -> Bool {
+  mutating func atStartOfGetSetAccessor() -> Bool {
     precondition(self.at(.leftBrace), "not checking a brace?")
 
     // The only case this can happen is if the accessor label is immediately after