bug(line): add missing layer 'crosshair' to TypeScript definition

[dancallaghan]

Resolves #862

[dancallaghan]

If anyone else is having this issue, in the meantime you can override it in your project:

// custom.d.ts
declare module '@nivo/line' {
    export type LineLayerType =
        | 'grid'
        | 'markers'
        | 'axes'
        | 'areas'
        | 'crosshair'
        | 'lines'
        | 'slices'
        | 'points'
        | 'mesh'
        | 'legends';
}

[plouc]

Thank you for your contribution @dancallaghan!