Instead of using core/snapshot_creator.rs, instead two crates are
introduced which allow building the snapshot during build.rs.

Rollup is removed and replaced with our own bundler. This removes
the Node build dependency. Modules in //js now use Deno-style imports
with file extensions, rather than Node style extensionless imports.

This improves incremental build time when changes are made to //js files
by about 40 seconds.

Closes #2720

This removes dispatch_flatbuffers as it is now unused. There are still a
few places where msg_generated is used: ErrorKind and MediaType. These
will be dealt with later.

* Revert "port more ops to JSON (#2809)"

This reverts commit 137f3373.

* Revert "port ops to JSON: compiler, errors, fetch, files (#2804)"

This reverts commit 79f82cf1.

* Revert "Port rest of os ops to JSON (#2802)"

This reverts commit 5b2baa5c.

Converts env(), exit(), execPath(), utime() and utimeSync() to use JSON
instead of flatbuffers.

Fixes #1432

Just some clean up reorganization around flatbuffer/minimal dispatch
code. This is prep for adding a JSON dispatcher.

This is causing a segfault for unknown reasons - see #2787.

This reverts commit 498f6ad4.

readSync and writeSync use dispatch_minimal now.

Note cli/dispatch_minimal.rs ops are not yet included in cli/ops.

This is part of work towards #2730