docs: loader hooks don't resolve jsr:/npm:/https: specifiers

[bartlomieju]

The Loader hooks reference didn't mention that external specifiers introduced
by hook-emitted source aren't resolved automatically. This trips people up when
they use a load hook to transpile a custom format (e.g. Civet, CoffeeScript)
whose output imports an npm or jsr package.

The reason is that Deno discovers and installs dependencies by statically
analyzing the module graph before execution, whereas hook-generated source is
produced at load time, after that analysis has finished. A bare jsr:, npm:,
or https: import that only appears in the emitted source is therefore invisible
to dependency resolution and fails with an error like
Could not find constraint 'lodash-es@latest' in the list of packages.

This adds an info box to the Custom transpilation use case documenting the
limitation, explaining why it happens, and showing the workaround of declaring
the dependency in deno.json up front. This is working as designed, so the docs
should set the expectation.

Refs denoland/deno#35896