net: add UDP listen, resolver, DNSError, and TCP listener APIs for wasm (#63)

* net: add UDP listen, resolver, DNSError, and TCP listener APIs for wasm

TinyGo's net package was missing several symbols that upstream Go's
js/wasm net declares, blocking builds that pull pion/transport, pion/dtls,
and similar (via netbird's WASM client). Add them, backed by the existing
netdev abstraction so they compile for all targets and no-op cleanly under
nopNetdev (matching Go's js runtime behavior):

- DNSError type (dnserror.go), copied from Go 1.26.2.
- Resolver + DefaultResolver with LookupHost/LookupIP/LookupIPAddr/
  LookupNetIP/LookupPort; LookupHost/LookupIP package funcs; Dialer.Resolver
  field for API compatibility.
- UDPConn: ListenUDP, ReadFromUDP(AddrPort), WriteToUDP(AddrPort),
  SetReadBuffer, SetWriteBuffer.
- TCPConn: CloseRead, SetNoDelay, SetReadBuffer, SetWriteBuffer,
  ReadFrom(io.Reader).
- TCPListener: ListenTCP, AcceptTCP, SetDeadline; ListenPacket package func.
- Interface: Addrs, MulticastAddrs stubs.

* more complete PR for adding more net support (#64)

* format unixsock

unixsock.go

@@ -6,6 +6,11 @@
 
 package net
 
+import (
+	"errors"
+	"time"
+)
+
 // BUG(mikio): On JS, WASIP1 and Plan 9, methods and functions related
 // to UnixConn and UnixListener are not implemented.
 
@@ -44,6 +49,33 @@ func (a *UnixAddr) opAddr() Addr {
 	return a
 }
 
+// UnixConn is an implementation of the Conn interface for connections to Unix
+// domain sockets.
+//
+// TINYGO: Unix sockets are not implemented (the browser has no filesystem
+// sockets). This type exists only to satisfy references and type assertions
+// such as those in github.com/gliderlabs/ssh agent forwarding; all methods
+// return an error. The corresponding code paths are never reached at runtime in
+// the wasm build.
+type UnixConn struct {
+	fd    int
+	laddr *UnixAddr
+	raddr *UnixAddr
+}
+
+var errUnixNotImplemented = errors.New("net: Unix sockets not implemented")
+
+func (c *UnixConn) Read(b []byte) (int, error)         { return 0, errUnixNotImplemented }
+func (c *UnixConn) Write(b []byte) (int, error)        { return 0, errUnixNotImplemented }
+func (c *UnixConn) Close() error                       { return errUnixNotImplemented }
+func (c *UnixConn) CloseRead() error                   { return errUnixNotImplemented }
+func (c *UnixConn) CloseWrite() error                  { return errUnixNotImplemented }
+func (c *UnixConn) LocalAddr() Addr                    { return c.laddr }
+func (c *UnixConn) RemoteAddr() Addr                   { return c.raddr }
+func (c *UnixConn) SetDeadline(t time.Time) error      { return errUnixNotImplemented }
+func (c *UnixConn) SetReadDeadline(t time.Time) error  { return errUnixNotImplemented }
+func (c *UnixConn) SetWriteDeadline(t time.Time) error { return errUnixNotImplemented }
+
 // ResolveUnixAddr returns an address of Unix domain socket end point.
 //
 // The network must be a Unix network name.