Error at new time if name is not usable

[itowlson]

Fixes #2052.

# Error cases from original issue
ivan@hecate:~/testing$ spin new -t http-rust 2023-11-spin
error: Invalid value "2023-11-spin" for '<NAME>': Each segment of the name must start with a letter. '2023', '11' do not start with a letter

ivan@hecate:~/testing$ spin new -t http-rust spin-2023-11
error: Invalid value "spin-2023-11" for '<NAME>': Each segment of the name must start with a letter. '2023', '11' do not start with a letter

ivan@hecate:~/testing$ spin new -t http-rust spin_2023_11
error: Invalid value "spin_2023_11" for '<NAME>': Each segment of the name must start with a letter. '2023', '11' do not start with a letter

# Singular form
ivan@hecate:~/testing$ spin new -t http-rust spin-2
error: Invalid value "spin-2" for '<NAME>': Each segment of the name must start with a letter. '2' does not start with a letter

# This is caught but the error message is not ideal: is this required to fix?
ivan@hecate:~/testing$ spin new -t http-rust _foo
error: Invalid value "_foo" for '<NAME>': Each segment of the name must start with a letter. '' does not start with a letter

# Accepted
ivan@hecate:~/testing$ spin new -t http-rust spin_y2023_m11

(add uses the same logic and gets the same errors.)

Long term it would be good to centralise this logic, but the new/add name rules are slightly more forgiving than the manifest parser (because the templates kebab-case the things we care about kebab-casing), so I didn't tackle that here.

[rylev]

Can the separator legally be any non-alphanumeric character? Is "foo😴a" a legal name?

[itowlson]

Well, it works, so I guess it's de facto legal! The name as written is only used for the app name, which Spin doesn't care about. Component names get kebab-cased during processing, which turns that into foo-a which is a legal kebab-id. And the generated directory gets snake-cased to foo_a which is a legal directory name.