Make Doug code owner of macros and Swift operators

While doing this, also move CODEOWNERS to .github folder

Author: ahoppen

CODEOWNERS renamed to .github/CODEOWNERS

@@ -7,7 +7,13 @@
 # Each line is a file pattern followed by one or more owners.
 # Order is important. The last matching pattern has the most precedence.
 
+# Macros
+Sources/_SwiftSyntaxMacros @DougGregor
+Tests/SwiftSyntaxMacrosTest @DougGregor
+
+# SwiftOperators
+Sources/SwiftOperators @DougGregor
+Tests/SwiftOperatorsTest @DougGregor
+
 # Owner of anything in SwiftSyntax not owned by anyone else.
-# N: Alex Hoppen
-# E: [email protected]
 * @ahoppen