build system web interface does not support Windows

[mlugg]

Split from #24588.

std.Build.WebServer is currently intentionally disabled on Windows hosts due to two standard library bugs:

• Writing to a socket on one thread while reading from it on another currently causes a peculiar deadlock. While official documentation doesn't explicitly state this, other resources online suggest that this should work as you would expect. This issue is probably because our current socket reading logic on Windows calls through to ReadFile rather than a WSA socket read function.
• The implementation of std.posix.writev on Windows currently triggers an infinite loop if a buffer has length 0, as it will always report zero-length writes.

Both of these bugs are expected to be fixed by not-yet-merged changes in the wrangle-writer-buffering branch, at which point this feature can be re-enabled (subject to appropriate testing of course).

[scottredig]

For reference, the first bug is #14233, with the root cause seemingly being identified in the responses.

[andrewrk]

Both of these bugs are expected to be fixed by not-yet-merged changes in the wrangle-writer-buffering branch, at which point this feature can be re-enabled (subject to appropriate testing of course).

merged in 5998a8c

[mlugg]

Fixed by #24774. I can confirm this works perfectly now -- thanks for the help @kcbanner!