Validate on `shopify theme share/pull/push/serve/dev` if users are running the command in a theme/empty directory

[karreiro]

Introduce a "Are you sure?" message when users:

• Run runningshopify theme share in a directory that doesn't seem [1] like a theme directory
• Run runningshopify theme serve/dev in a directory that doesn't seem [1] like a theme directory
• Run runningshopify theme push in a directory that doesn't seem [1] like a theme directory
• Run runningshopify theme pull in a directory that doesn't seem [1] like a theme directory or it's not empty

[1] https://shopify.dev/themes/architecture#directory-structure-and-component-types

[karreiro]

It's related to https://github.com/Shopify/shopify-cli/issues/2623