This commit changes the `deno jupyter` subcommand:
- `deno jupyter` now accepts additional `--name` argument to
allow installing and maintaing multiple kernelsspec - useful when
one wants to install a stable kernel and a debug/canary kernel
- `deno jupyter --install` now accepts additional `--display`
argument to allow customizing display name of the kernel - the
default one is "Deno"
- `deno jupyter --install` no longer blindly installs kernelspec,
instead it first checks if a kernelspec already exists and if so,
returns an error suggesting to use `--force` flag
- `deno jupyter --help` no longer shows `--unstable` flag
Closes https://github.com/denoland/deno/issues/29219
Closes https://github.com/denoland/deno/issues/29220
