`jsr add` command should be able to install packages globally with `-g` or `--global` CLI flag

## What is the issue?

Currently these commands fail:
- `npx jsr add -g <pkg>`
- `bunx jsr add -g <pkg>`
- `jsr add -g <pkg>`

## Why do we need it?

Some CLI "bin" packages need to be used installed globally. `jsr` CLI program itself should be installed globally via `-g` or `--global` flag.

Similar issue for `deno add -g` posted here: https://github.com/jsr-io/jsr/issues/213