tests(init): add flag conflict spec tests
Adds characterization spec tests for the mutually-exclusive project-type
flags of `deno init`. The following pairs are rejected by clap:
- `--npm` with `--jsr`, `--lib`, `--serve`, or `--empty`
- `--jsr` with `--lib`, `--serve`, or `--empty`
- `--serve` with `--lib`
- `--empty` with `--lib` or `--serve`
The existing init specs only cover the happy paths (`--lib`, `--npm`,
`--yes`); these constraints had no coverage. Locking in the current
behavior guards against accidentally dropping a conflict when the
subcommand definition is refactored.
