esbuild 0.16.3

Fix a hang with the JS API in certain cases (#2727)

A change that was made in version 0.15.13 accidentally introduced a case when using esbuild's JS API could cause the node process to fail to exit. The change broke esbuild's watchdog timer, which detects if the parent process no longer exists and then automatically exits esbuild. This hang happened when you ran node as a child process with the stderr stream set to pipe instead of inherit, in the child process you call esbuild's JS API and pass incremental: true but do not call dispose() on the returned rebuild object, and then call process.exit(). In that case the parent node process was still waiting for the esbuild process that was created by the child node process to exit. The change made in version 0.15.13 was trying to avoid using Go's sync.WaitGroup API incorrectly because the API is not thread-safe. Instead of doing this, I have now reverted that change and implemented a thread-safe version of the sync.WaitGroup API for esbuild to use instead.