add type annotations to toolkit.nu and fix a bug

[amtoine]

this PR adds some type annotations to the toolkit.nu and fixes a bug where running

tk run --interactive

for the first time would give an error

Error:   × Permission denied
    ╭─[/home/amtoine/documents/repos/github.com/amtoine/nu-git-manager/toolkit.nu:58:1]
 58 │
 59 │         "$env.config = {show_banner: false}" | save --force $CONFIG_FILE
    ·                                                             ──────┬─────
    ·                                                                   ╰── No such file or directory (os error 2)
 60 │         "" | save --force $ENV_FILE
    ╰────