TypeScript strictNullChecks compatibility

[BenoitZugmeyer]

Is your feature request related to a problem? Please describe.

I want to use bumbag in a TS project configured with "strict": true or "strictNullChecks": true, but bumbag types are not compatible.

Describe the solution you'd like

Adjust bumbag types to be compatible with strictNullChecks.

Describe alternatives you've considered

I could also disable strictNullChecks in my project, but that's really not ideal.

Additional context

Please see this gist to reproduce the issue.

TS output:

node_modules/bumbag/ts/types/theme.d.ts:95:5 - error TS2411: Property 'default' of type '{ color: string; width: string; } | ((props: { theme: ThemeConfig; }) => { color: string; width: string; }) | undefined' is not assignable to string index type 'ThemeAttribute<{ color: string; width: string; }>'.

95     default?: ThemeAttribute<{
       ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:105:5 - error TS2411: Property 'default' of type 'string | ((props: { theme: ThemeConfig; }) => string) | undefined' is not assignable to string index type 'ThemeAttribute<string>'.

105     default?: ThemeAttribute<string>;
        ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:120:5 - error TS2411: Property 'importUrls' of type 'string[] | undefined' is not assignable to string index type 'string | string[]'.

120     importUrls?: Array<string>;
        ~~~~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:121:5 - error TS2411: Property 'default' of type 'string | undefined' is not assignable to string index type 'string | string[]'.

121     default?: string;
        ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:122:5 - error TS2411: Property 'heading' of type 'string | undefined' is not assignable to string index type 'string | string[]'.

122     heading?: string;
        ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:123:5 - error TS2411: Property 'mono' of type 'string | undefined' is not assignable to string index type 'string | string[]'.

123     mono?: string;
        ~~~~

node_modules/bumbag/ts/types/theme.d.ts:148:5 - error TS2411: Property 'minorUnit' of type 'number | ((props: { theme: ThemeConfig; }) => number) | undefined' is not assignable to string index type 'ThemeAttribute<string | number>'.

148     minorUnit?: ThemeAttribute<number>;
        ~~~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:149:5 - error TS2411: Property 'majorUnit' of type 'number | ((props: { theme: ThemeConfig; }) => number) | undefined' is not assignable to string index type 'ThemeAttribute<string | number>'.

149     majorUnit?: ThemeAttribute<number>;
        ~~~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:156:5 - error TS2411: Property 'modes' of type '{ [key: string]: PaletteThemeConfig; } | undefined' is not assignable to string index type 'string | { [key: string]: PaletteThemeConfig; }'.

156     modes?: {
        ~~~~~

node_modules/bumbag/ts/types/theme.d.ts:826:9 - error TS2411: Property 'info' of type 'string | undefined' is not assignable to string index type 'string'.

826         info?: string;
            ~~~~

node_modules/bumbag/ts/types/theme.d.ts:827:9 - error TS2411: Property 'warning' of type 'string | undefined' is not assignable to string index type 'string'.

827         warning?: string;
            ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:828:9 - error TS2411: Property 'success' of type 'string | undefined' is not assignable to string index type 'string'.

828         success?: string;
            ~~~~~~~

node_modules/bumbag/ts/types/theme.d.ts:829:9 - error TS2411: Property 'danger' of type 'string | undefined' is not assignable to string index type 'string'.

829         danger?: string;
            ~~~~~~