Diagnostic when someone uses a wildcard in an export

[dsherret]

denoland/deno#26693

[satyarohith]

Shouldn't this be an error instead of a diagnostic similar to

deno_config/src/deno_json/mod.rs

Lines 1066 to 1071 in ca59535

	if key.is_empty() {
	bail!(
	"The {} must not be empty. Use '.' if you meant the root export.",
	key_display()
	);
	}

?

Edit: there is already an error that is generated for unexpected characters:

deno_config/src/deno_json/mod.rs

Lines 2348 to 2351 in ca59535

	run_test(
	r#"{ "exports": { "./mod*": "./mod.ts" } }"#,
	"The './mod*' export must only contain alphanumeric characters, underscores (_), dashes (-), dots (.), and slashes (/).",
	);