Fix quadratic CSVFormat.printWithEscapes(Reader) delimiter look-ahead

[rootvector2]

the reader overload of printWithEscapes rebuilt a string from the whole accumulated output on every character to run the delimiter look-ahead, so escaping a large Reader value with no escapable characters ran in O(n^2). found while benchmarking format.print(Reader, out, true) in an escape format like MYSQL: a 0.8 MB field took ~28 s versus ~6 ms for the quoted path. the look-ahead only needs the current character and the peeked buffer, so test is built from those instead of the whole builder, which is the same match but linear (~19 ms at 0.8 MB).

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[garydgregory]

Good one @rootvector2 , thank you & merged 🚀