Add "Die On: warning" to spec metadata

[inexorabletash]

Although not yet documented, Bikeshed now supports setting the default --die-on in the spec metadata. We should make warnings fatal so contributors don't need to remember to specify --die-on=warning manually.

This should just require adding this to the <pre class='metadata'> block at the top of the spec:

Die On: warning

Also, I'm explicitly suggesting "warning" rather than "link-error" because the spec is currently clean for warnings and most Bikeshed warnings really do flag that something is wrong, so we don't want to regress.

[anssiko]

@inexorabletash, great idea!

@dontcallmedom, wouldn't a simple BUILD_FAIL_ON: warning in https://github.com/webmachinelearning/webnn/blob/main/.github/workflows/auto-publish.yml#L20C32-L20C32 address this?

(I note the spec-prod example workflow https://github.com/w3c/spec-prod/blob/main/docs/examples.md does not define any exit behaviour on build errors. Should this default be liberal or not? Just wondering.)

[dontcallmedom]

BUILD_FAIL_ON: warning should work according to https://github.com/w3c/spec-prod/blob/958b6d59f51720a7b0f2d402f87ba193e39bc080/docs/options.md#build_fail_on

[anssiko]

@inexorabletash, maybe we should also fail the CI build similarly as in #469? The in-spec metadata directive will catch local build experiments.

[inexorabletash]

Yes, failing CI is also a good idea.