refactor(runtime): conditionally register Extension with source files (#18068)

Since we are snapshotting extension source at build time, there's no
need to define list of sources for the extension at runtime.

This commit changes "deno_node" extension by removing "init_polyfill"
function in favor of "init_polyfill_ops_and_esm()" and "init_polyfill_ops()".

The former is used during snapshot and when "deno_runtime" is compiled
with "dont_create_runtime_snapshot" cargo feature flag. The latter is used
when running a worker from an existing snapshot.

This is a start of a bigger refactor to all extensions - thanks to this
change, we don't have to iterate over all defined source files for extension at
runtime, and because of that we don't have to create a filepath for each of the
source files. It's not a big deal, but we are iterating over 300 files on each start,
and concatenating 3 strings before creating a "PathBuf" for ~200 of them.
This is already visible on the startup flamegraphs and should be avoided.

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