move `add` -> `template add` and `module add` -> `add`

[danielroe]

adding new modules is probably more common than adding templates, and it would be nice to prioritise this.

# add new module
pnpm nuxt add fonts

# add new page
pnpm nuxt template add page

it's technically a 'breaking change' but it should be a reasonably straightforward one to get to grips with

I'd love feedback from the community first, though! 🙏

[atinux]

I think we should also support nuxt module add until all modules update their documentation (probably adding a warning and run the nuxt add command)