AstPrinter produces invalid GraphQL #3229
Description
Describe the bug
When a non-BlockString string contains escaped characters like newlines or quotes, AstPrinter does not escape them and the resulting document is not valid GraphQL.
=== Original ===
"\"" scalar A
=== Parsed and printed ===
"""
scalar A
Exception in thread "main" graphql.parser.InvalidSyntaxException: Invalid syntax with ANTLR error 'token recognition error at: '"""\nscalar A\n'' at line 1 column 1
at graphql.parser.Parser$1.syntaxError(Parser.java:368)
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