Remove flatbuffers compile support

[ry]

This commit checks into the repo the two generated files
cli/msg_generated.rs
js/msg_generated.js
And removes support for automatically building them from the build.

This is further simplifications in order to allow the cli crate to build
without gn/ninja. See #2608.

[ry]

Beware everyone - this PR marks the beginning of the end for flatbuffers (See #2121). It removes support for auto-generating msg_generated.ts and msg_generated.rs, so ease refactoring of the build system.

The two major hurdles of getting cli off gn/ninja is flatbuffers and snapshots. This takes care of the flatbuffer problem. The snapshot solution is more tricky and being worked on over at https://github.com/ry/deno_typescript ... I plan to merge deno_typescript work into this repo soon.

> git log --shortstat --pretty=oneline master...
733cb3a988a3607fee81733f0c837ad429102492 (HEAD -> remove_fbs, ry/remove_fbs) Remove flatbuffers compile support
 46 files changed, 67 insertions(+), 535 deletions(-)
ddaa997fa92bd4565d953ef72e54f3c81afbe421 Add generated flatbuffer code to repo
 3 files changed, 20992 insertions(+)

[piscisaureus]

Any guidance on how to run flatbuffers manually should it be needed?
Or are JSON messages close enough for this to never be needed?

[ry]

Porting ops to JSON is actually going smoothly. I want to delay ripping out the build support for flatbuffers, as I think we can do it better gradually as in #2799 ...

I hope to re-apply this in a few days but without having to add the generated files to repo.