Allow trailing comma in enum value lists, per spec

[motiz88]

See whatwg/webidl@291dcc3

[jyasskin]

Yep, https://heycam.github.io/webidl/#idl-enums allows trailing commas when EnumValueListString expands to ε.

[19]    Enum    → "enum" identifier "{" EnumValueList "}" ";"
[20]    EnumValueList   → string EnumValueListComma
[21]    EnumValueListComma  → "," EnumValueListString 
                            | ε
[22]    EnumValueListString → string EnumValueListComma 
                            | ε