Add tracing support to tsserver (#41374)

* Add tracing support to tsserver

Read the `TSS_TRACE` environment variable to determine which directory
trace files should be written to.

Notable changes from tsc tracing:
1) Drop all tracepoints that depend on type IDs
2) Write output to trace.PID.json
3) New, server-specific events (request/response, cancellation, etc)

* Drop try-finally blocks that aren't strictly necessary

* Fix lint error

* Trace background work (for diagnostics)

* Move try-finally blocks into session so tsc doesn't use them

* Add missing try-finally

* Use consistent capitalization

* Inline canPop call where underlying variable is available

* Clarify comments

* Include PID in build-mode file names

* Introduce more efficient popAll function

* Trace throwIfCancellationRequested rather than isCancellationRequested

* Remove unnecessary try-finally blocks

* Add a command-line argument for consistency with logging

* Fix rebase issues

* Address PR feedback

* Rename completionEvents to eventStack

* Drop assertStackEmpty as hard-to-maintain and marginally valuable

* Rename stepCancellation to stepCanceledEarly

* Rename stepEarlyCancellation to stepCanceled and use flag instead

* Check correct variable on exit

src/executeCommandLine/executeCommandLine.ts

@@ -621,7 +621,7 @@ namespace ts {
         }
 
         if (canTrace(system, compilerOptions)) {
-            tracing.startTracing(compilerOptions.configFilePath, compilerOptions.generateTrace!, isBuildMode);
+            tracing.startTracing(isBuildMode ? tracing.Mode.Build : tracing.Mode.Project, compilerOptions.generateTrace!, compilerOptions.configFilePath);
         }
     }