Add support for sourcemap *.map files

[nolanlawson]

For some apps I'd like to enable source maps in production. However this causes the following output for %sapper.scripts%:

<script src='/client/<hash>/main.js'></script>
<script src='/client/<hash>/main.js.map'></script>

This PR fixes it so that Sapper outputs:

<script src='/client/<hash>/main.js'></script>

[nolanlawson]

I realized my previous PR was incomplete. This new commit adds full support for .map files, for both sapper start and sapper export, for both the client and the service worker.

So now a user can, for instance, put the following configuration into client.config.js or service-worker.config.js:

devtool: isDev ? 'inline-source-map' : 'source-map'

...and all the *.map files will be correctly served and exported.

[nolanlawson]

Last change: there's no sense in adding <link rel=preload> for .map files.

[Rich-Harris]

sweet, thanks! cutting a release shortly