Use dependabot to automate rust dependencies update

[F3n67u]

Would deno interested in using dependabot for automating rust dependencies update?

I already setup my fork deno repo for dependabot. If this idea is accepted, I am happy to make a pr for dependabot setup.

Commit:

F3n67u@cff79d3

Example deps update pr:

https://github.com/F3n67u/deno/pulls

[F3n67u]

I have already open a pr for this idea: #11922