Improve debug I/O color detection/handling for WASI and Emscripten

When linking libc, these targets can get their environment variables
from `std.c.environ`. Additionally, it's okay for WASI to use ANSI
escape sequences; nothing in the relevant specs claim otherwise.

lib/std/Io/Threaded.zig

@@ -8729,13 +8729,6 @@ fn supportsAnsiEscapeCodes(file: File) Io.Cancelable!bool {
         }
     }
 
-    if (native_os == .wasi) {
-        // WASI sanitizes stdout when fd is a tty so ANSI escape codes will not
-        // be interpreted as actual cursor commands, and stderr is always
-        // sanitized.
-        return false;
-    }
-
     if (try isTty(file)) return true;
 
     return false;

lib/std/process/Environ.zig

@@ -24,7 +24,7 @@ pub const empty: Environ = .{ .block = .empty };
 /// operating system `void` is also used.
 pub const Block = switch (native_os) {
     .windows => GlobalBlock,
-    .wasi => switch (builtin.link_libc) {
+    .wasi, .emscripten => switch (builtin.link_libc) {
         false => GlobalBlock,
         true => PosixBlock,
     },

lib/std/start.zig

@@ -664,10 +664,22 @@ fn main(c_argc: c_int, c_argv: [*][*:0]c_char, c_envp: [*:null]?[*:0]c_char) cal
 
 fn mainWithoutEnv(c_argc: c_int, c_argv: [*][*:0]c_char) callconv(.c) c_int {
     const argv = @as([*][*:0]u8, @ptrCast(c_argv))[0..@intCast(c_argc)];
-    if (@sizeOf(std.Io.Threaded.Argv0) != 0) {
-        if (std.Options.debug_threaded_io) |t| t.argv0.value = argv[0];
+    const environ: [:null]?[*:0]u8 = switch (builtin.os.tag) {
+        .wasi, .emscripten => environ: {
+            const c_environ = std.c.environ;
+            var env_count: usize = 0;
+            while (c_environ[env_count] != null) : (env_count += 1) {}
+            break :environ c_environ[0..env_count :null];
+        },
+        else => &.{},
+    };
+    const env_block: std.process.Environ.Block = .{ .slice = environ };
+    if (std.Options.debug_threaded_io) |t| {
+        if (@sizeOf(std.Io.Threaded.Argv0) != 0) t.argv0.value = argv[0];
+        t.environ = .{ .process_environ = .{ .block = env_block } };
+        t.environ_initialized = env_block.isEmpty();
     }
-    return callMain(argv, .empty);
+    return callMain(argv, env_block);
 }
 
 /// General error message for a malformed return type