feat(upgrade): support deno upgrade 1.46.0

[bartlomieju]

This commit changes deno upgrade subcommand to accept
a positional argument that can be either a version, release channel
name or a git hash, making invocations of deno upgrade much
more concise:

# before
$ deno upgrade --version 1.46.0
# after 
$ deno upgrade 1.46.0

# before
$ deno upgrade --canary
# after
$ deno upgrade canary

# specific canary version before
$ deno upgrade --canary --version f042c39180c1b345de8e7b5f0dfae5d0a49b161f
# after
$ deno upgrade f042c39180c1b345de8e7b5f0dfae5d0a49b161f

Old flags are still supported, but hidden from the help output.

[bartlomieju]

IMPORTANT: Update doc page before landing

Actually I'm gonna give it a couple weeks to let users upgrade to the version that supports this syntax before updating docs.

[dsherret]

LGTM