sysfs: adds chmod (#1135)

This adds `FS.Chmod` and implements it for `GOOS=js`. This function
isn't defined in WASI snapshot01, but it is in `wasi-filesystem`, e.g.
`change-file-permissions-at`.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/gojs/errno_test.go

@@ -13,6 +13,11 @@ func TestToErrno(t *testing.T) {
 		input    error
 		expected *Errno
 	}{
+		{
+			name:     "syscall.EACCES",
+			input:    syscall.EACCES,
+			expected: ErrnoAcces,
+		},
 		{
 			name:     "syscall.EAGAIN",
 			input:    syscall.EAGAIN,