Use fssnip highlighter in API docs code examples.

[nhirschey]

This adds syntax highlighting to API docs code examples. PR #762 from @yazeedobaid was solving the same problem, but it relied on external tools such as highlight.js to do the highlighting and required users to add that dependency. This PR uses FSharp.Formatting's baked-in highlighter to achieve the same goal.

For example, building the F# core docs Array.averageBy example using this PR:

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In comparison, this is how it looks now:

[nhirschey]

@yazeedobaid are you happy with this version?

[nhirschey]

Now with the extra trailing blank line after the last line of code removed (adding back a prior dsyme fix). Should be ready for release.

[yazeedobaid]

@nhirschey tested the branch with the FAKE website and all is good. But I may add back that padding by a CSS rule for the code examples since it is more visually looking good with some padding:

That is FAKE related. Thanks for the update.

[dsyme]

@nhirschey Fantastic work, thank you!