x/pkgsite: add support for syntax highlighting of code blocks

[codyoss]

What is the URL of the page with the issue?

Any containing code blocks. Example: https://pkg.go.dev/cloud.google.com/go/storage?tab=doc#hdr-Objects

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.135 Safari/537.36

Screenshot

What did you do?

Navigate to the page.

What did you expect to see?

Code blocks with syntax highlighting.

What did you see instead?

Code blocks as plain text.

[julieqiu]

Thanks for the suggestion, @codyoss!

/cc @spf13 @Joanne881107 @fflewddur for product / UX input

[dmitshur]

Note that there also exist some preformated text blocks that don't contain Go code. For example:

(Source: https://pkg.go.dev/cmd/asm#hdr-Command_Line)

[codyoss]

Note that there also exist some preformated text blocks that don't contain Go code

That is a very good point I had not thought of... Might make this quite tricky(infeasible?) as there is no way today to call out what is contained in the block.(i.e. the way markdown supports "```go")