chore(syncthing): include runtime context in GC crashes (#10702)

The runtime prints a lot of context for crashes due to bad pointers etc,
which is required to understand the crash, but this context comes before
the `fatal error: ...` line. Currently those lines get filtered out and
not included in the crash report. This change modifies the criteria so
that we start collecting crash data also at a line that begins with
`runtime:`, and tweaks the parsing later to look for the specific
`panic:` or `fatal error:` which may come later as the subject.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

cmd/syncthing/monitor.go

@@ -233,7 +233,7 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 
 		dst.Write([]byte(line))
 
-		if panicFd == nil && (strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:")) {
+		if panicFd == nil && (strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:") || strings.HasPrefix(line, "runtime:")) {
 			panicFd, err = os.Create(locations.GetTimestamped(locations.PanicLog))
 			if err != nil {
 				slog.Error("Failed to create panic log", slogutil.Error(err))