webstead: use a non-module <script> and import()

webstead: use a non-module <script> and import()

According to <https://github.com/fuweichin/xhtml5-esm-support>, an
internal script (a <script> element with content) can't be a module in
an XHTML page in Safari.  So following the suggestion on that page, I
make the <script> on the page into a classic script, and use import()
to load the module containing the bedstead executable.  Maybe this
will work on a reasonable selection of modern browsers.