std: rework TTY detection and printing

This commit sketches an idea for how to deal with detection of file
streams as being terminals.

When a File stream is a terminal, writes through the stream should have
their escapes stripped unless the programmer explicitly enables terminal
escapes. Furthermore, the programmer needs a convenient API for
intentionally outputting escapes into the stream. In particular it
should be possible to set colors that are silently discarded when the
stream is not a terminal.

This commit makes `Io.File.Writer` track the terminal mode in the
already-existing `mode` field, making it the appropriate place to
implement escape stripping.

`Io.lockStderrWriter` returns a `*Io.File.Writer` with terminal
detection already done by default. This is a higher-level application
layer stream for writing to stderr.

Meanwhile, `std.debug.lockStderrWriter` also returns a `*Io.File.Writer`
but a lower-level one that is hard-coded to use a static single-threaded
`std.Io.Threaded` instance. This is the same instance that is used for
collecting debug information and iterating the unwind info.

lib/std/Progress.zig

@@ -755,10 +755,9 @@ fn appendTreeSymbol(symbol: TreeSymbol, buf: []u8, start_i: usize) usize {
     }
 }
 
-fn clearWrittenWithEscapeCodes(w: *Io.Writer) anyerror!void {
+pub fn clearWrittenWithEscapeCodes(file_writer: *Io.File.Writer) anyerror!void {
     if (noop_impl or !global_progress.need_clear) return;
-
-    try w.writeAll(clear ++ progress_remove);
+    try file_writer.interface.writeAllUnescaped(clear ++ progress_remove);
     global_progress.need_clear = false;
 }