Add schema directive to empty template

[itowlson]

Fixes #3699.

The current AppManifest serialisation type requires that there be at least one trigger. This makes the empty manifest invalid under the current schema.

I've proposed to fix this by allowing an empty set of triggers at the serialisation level, but validating during load. This changes the error output when you spin up an app with no triggers:

# Before

$ spin up
Error: TOML parse error at line 1, column 1
  |
1 | spin_manifest_version = 2
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
missing field `trigger`

# After

$ spin up
Error: Failed to load application

Caused by:
   0: Failed to load manifest from "spin.toml"
   1: Failed to load Spin app from "spin.toml"
   2: The application must have at least one trigger

The alternative is to keep the existing requirement and accept that there will be red squigglies in a new empty app. However, like the "before" error, the message will be correct but it will point to the wrong place, so I have pretty mixed feelings about this.

Note that we will want to publish a new schema directive before tagging this template - at the moment we merge it, the directive will continue to point to the "trigger required" schema, which fails. But as noted, the failure at least comes with a good error message, so eh.

[itowlson]

Pleasingly, this also fixes #3698 does the bug smiting dance