Processing shebang and byte order mark

Shebang

§18.1 (fls_s35hob3i7lr) provides the following syntax definitions:

SourceFile ::=
    ZeroWidthNoBreakSpace?
    Shebang?
    InnerAttributeOrDoc*
    Item*

Shebang ::=
    #! ~[NewLine]*

NewLine ::=
    \n

This grammar is ambiguous: a file starting with the following could be treated as starting with either a Shebang or an InnerAttributeOrDoc:

#![xxx]

This is an unusual case where the ambiguity isn't resolved "eagerly": rustc sees this as InnerAttributeOrDoc. So I think the FLS ought to say more here.

Also, I think putting a nonterminal inside ~[] isn't strictly following the notation described by §1.1.4:14 (fls_blvsfqeevosr). The definition of NewLine isn't used anywhere else.

Byte order mark

§18.1:2 (fls_ann3cha1xpek) reads

A Shebang does not have an effect on the compilation.

I suggest adding a similar statement for the ZeroWidthNoBreakSpace.

Start with §18.1 and compare its SourceFile, Shebang, and NewLine definitions with the notation rules in §1.1.4:14. Check rustc's handling of #![xxx] and the leading zero-width no-break space, then update the relevant FLS text so the grammar and compilation-effect statements match the intended behavior.