[naga wgsl-in] Add directive keywords to "expected" values on global item parsing

[ErichDonGubler]

Is your feature request related to a problem? Please describe.

#6352 adds/added parsing for directives, which only happen before global declarations. However, it did not add any diagnostic adjustments for the final, fatal diagnostic presented when Naga simply did not find any sentinel tokens indicating a deeper parse path to try than Parser::global_decl. To be painfully concrete, I'm referring to the code here:

wgpu/naga/src/front/wgsl/error.rs

Lines 340 to 343 in db334f7

	ExpectedToken::GlobalItem => concat!(
	"global item ('struct', 'const', 'var', 'alias', ';', 'fn') ",
	"or the end of the file"
	)

Describe the solution you'd like

We should also suggest directive identifiers in the set of sentinels that begin a global item for this diagnostic.

Describe alternatives you've considered

We could simply not do this. However, this feels like hiding functionality in a way that hurts, rather than helps, users.

Additional context

-

[jimblandy]

This just a matter of extending the list of tokens in the error message to cover what's actually permitted.