[9.x] feature: Str::inlineMarkdown()

[danharrin]

Currently, Str::markdown() wraps all content in a block element <p> when converting.

For certain layouts, this is inconvenient, as the markdown needs to be rendered inline instead.

Commonmark has a built-in InlinesOnly extension that we can use in a Str::inlineMarkdown() method.

Additionally, I've added an inlineMarkdown() method to the fluent Stringable object to maintain consistency.

Str::markdown('Use [Laravel](https://laravel.com)!') // <p>Use <a href="https://laravel.com">Laravel</a>!</p>

Str::inlineMarkdown('Use [Laravel](https://laravel.com)!') // Use <a href="https://laravel.com">Laravel</a>!

[danharrin]

@taylorotwell do you want a docs PR for this? or are you on it already

[driesvints]

@danharrin feel free to send in a PR to the docs 👍

[danharrin]

laravel/docs#8039

[driesvints]

@danharrin thanks!