http2: [compat api] add writable* properties
added writableHighWaterMark, writableLength, and writableFinished properties with test.
Refs: https://github.com/nodejs/node/issues/29829 (9th, 10th, and 11th sub-issues)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines