Preserve Formatting of Symbol Declarations

[mattt]

Currently, code declarations for symbols are generated from the description of SwiftSemantics objects constructed from syntax nodes. However, this removes any inter-node trivia like newlines that may help break up long lines.

After considering running the code through a formatter like swift-format, I decided that a better approach would be to preserve the existing whitespace, working on the assumption that developers will adopt formatting conventions that look good in their editor.

This PR also tweaks the CSS rules for declarations to better accommodate standard 80 character column widths. Lines up to that width should display without a line break when viewed on desktop browsers at the standard text scale. Longer lines will now wrap according to pre-wrap behavior (see #144 (comment)), instead of overflowing with a scroll bar.

Resolves #201

Before

After

[mattt]

One last thing worth mentioning: This change has the incidental benefit of reducing a String -> AST -> HTML conversion step. The current / old way of doing things involved doing a syntactic parse in SourceFile to collect symbols, and then doing a separate parse in the HTML post-processing step to do syntax highlighting. With this PR, we now feed the AST nodes from SourceFile directly into SwiftSyntaxHighlighter, which should make things run a bit faster.

[MaxDesiatov]

Seems legit 👍