preserve npm sourcemaps

[Fil]

The easiest of the many things we can/want/might do with sourceMaps:

• preserve sourceMap URL for npm modules, as links to jsdeliver

For example, at the bottom of http://127.0.0.1:3000/_npm/[email protected]/_esm.js this PR now shows:

//# sourceMappingURL=https://cdn.jsdelivr.net/sm/42676edc1731d21c3307f50afea3dbba6be989c9c9d267599113a48e3ca85bf9.map

But, is this something we would only want to do in preview? We could censor the sourceMappingURL=… lines in the build.

Related issues:

• preserve and copy sourceMaps for user code (#832)
• create sourceMaps from user code (#805), at least in preview