✨feat(theme): adds theme prop to components, new store, and helper components

[dysfunc]

Why these changes?
This includes:

• Adds theme prop to all container components for scoped theming
• Adds a new theme store hook to control global theming
• Adds a <Theme> and <ThemeToggler> as additional helper components to control theming (scoped + global)

Theme prop

<script>
  import { Alert } from '@sveltestrap/sveltestrap';
</script>

<Alert theme="dark" ... />

Theme Provider

<script>
  import { Theme } from '@sveltestrap/sveltestrap';
</script>

<Theme theme="dark">
  ...anything
</Theme>

Theme store

<script>
  import { colorMode, Button } from '@sveltestrap/sveltestrap';

  const onClick = () => {
    $colorMode = 'dark';
  };
</script>

<Button on:click={onClick}>Switch to dark mode</Button>

Theme Hook

<script>
  import { useColorMode, Button } from '@sveltestrap/sveltestrap';

  const onClick = () => {
    useColorMode('dark');
  };
</script>

<Button on:click={onClick}>Change global mode to dark</Button>

The theme hook will accept an element ref. If a valid ref has been provided, it will apply the theme change to that element only - not globally.

Closes #33

[bestguy]

Awesome looks great, thanks for all the work with all these files.