Add MonitoringStream.DidFlush event

[AArnott]

Closes #131

[codecov]

Codecov Report

Merging #149 into master will increase coverage by 3.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   87.35%   90.66%   +3.31%     
==========================================
  Files          33       34       +1     
  Lines        2380     2433      +53     
==========================================
+ Hits         2079     2206     +127     
+ Misses        301      227      -74

Impacted Files	Coverage Δ
src/Nerdbank.Streams/MonitoringStream.cs	100% <100%> (ø)	⬆️
src/Nerdbank.Streams/Substream.cs	100% <0%> (ø)	⬆️
src/Nerdbank.Streams/SpanPolyfillExtensions.cs	30% <0%> (ø)
src/Nerdbank.Streams/MultiplexingStream.cs	91.19% <0%> (+0.95%)	⬆️
src/Nerdbank.Streams/BufferTextWriter.cs	88.04% <0%> (+1.08%)	⬆️
src/Nerdbank.Streams/Utilities.cs	97.26% <0%> (+1.36%)	⬆️
src/Nerdbank.Streams/MultiplexingStream.Channel.cs	92.43% <0%> (+10.08%)	⬆️
src/Nerdbank.Streams/PipeExtensions.cs	99.1% <0%> (+16.14%)	⬆️
src/IsolatedTestHost/TestOutputHelper.cs	50% <0%> (+50%)	⬆️
src/IsolatedTestHost/Program.cs	59.42% <0%> (+59.42%)	⬆️

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