Add a new check to flag use of const enum in declaration files

[mhegazy]

const enums break single file compilation. we should add a check to flag them and make sure no new ones are added at least.

[zspitz]

const enums break single file compilation

@mhegazy Could you elaborate on this? Ambient const enums are useful in documenting fixed values not made available by the library's object model.

[ghost]

For reference, looks like that question was answered in microsoft/TypeScript#20703