Returns the file extension of the file at a given path.
If the file has no extension (eg Makefile, etc), then '' will be
returned.
@parampathOrFilename — The input path@paramoptions — Options which affect the return value. See ExtnameOptions.
declare function extname(
pathOrFilename: string | Path,
options?: ExtnameOptions,
): string;Options for extname and Path.prototype.extname.
declare interface ExtnameOptions {
full?: boolean;
}Whether to get compound extensions, like .d.ts or .test.js, instead of
just the final extension (.ts or .js in this example).
full?: boolean;