Validate name when instantiating template

[itowlson]

Fixes #739.

Output:

$ spin new http-rust 123test
error: Invalid value "123test" for '<NAME>': Name must start with a letter

Notes for reviewers:

1. This validates in the CLI, rather than in the template crate itself. Is this reasonable or should we push the validation into that shared core?

2. This makes no consideration for language-specific naming rules. E.g. if a language used the name only for directory and component names, this restriction would be unnecessary. Is this okay or should we move the validation somewhere that understands per-language or per-template constraints?

3. This forbids problematic names. An alternative approach would be to transform them into legal ones, which could be done via Liquid filters in the templates (again allowing more language-specific understanding) - e.g. transform 34 to spinapp34. What do we prefer?

Signed-off-by: itowlson ivan.towlson@fermyon.com

[fibonacci1729]

Looks good to me.

[kate-goldenring]

I think it makes sense to catch it with clap for the time being. If other languages allow non-letter first names and an issue comes up around that, then we could switch, but prioritizing early and simple checking seems appropriate. LGTM!