Added option to render styles with optional separator

[SuperJura]

The problem with the current implementation is that you cannot modify the rendering of the styles when you have multiple lines to render.
The only way to render multilines is having "\n" between the lines. That is not only cumbersome, but has bugged view with extra space character at the start of every new line.
This PR fixes this.

this is how it looks when rendering code
lines = []string{ "first line", "\n", "second line", }

this is how it looks when rendering PR code
lines = []string{ "first line", "second line", } ... style.RenderWithSeparator("\n", lines...),

[SuperJura]

This can also be done with current code and passing one string with "\n" or some other separator, but if the API already supports multiple lines, this feels like correct way to connect the lines.

[meowgorithm]

Hi! Thanks for this—it's interesting and your implementation is great. We're going to consider it, though do note that we're generally cautious about increasing API surface area.

[SuperJura]

great, let me know what you decided.