RFC: Start deno process with DENO_FUTURE=1 environment variable

[lambdalisue]

Is your feature request related to a problem?

No

Describe the solution you'd like

To prepare supporting Deno v2, we should use DENO_FUTURE=1 environment variable.
https://deno.com/blog/v1.42#deno_future1-environment-variable

Additional context

This will remove tons of things from Deno and probably old denops plugins will stop working. So we should add "compatible mode" or whatever?

How to test

DENO_FUTURE=1 nvim

or

DENO_FUTURE=1 vim