Improve highlighted code samples in blog-index.html

* Get rid of the first and last empty lines in each sample
* Indent everything uniformly with 2 or 4 spaces
* Display the syntax highlighting of numeric constants with vacuous if
  statements. Numbers are quite common in coding, and in a lot of the
  languages, they have their own highlighted style; we should let people
  see what they look like. This also provides an excuse to show off
  control structure keyword highlighting in the bare JavaScript sample
  and importing something in Haskell.
* Display other syntax elements, notably in CSS:
  - ID, class, pseudo-element selectors
  - string and measurement and built-in literals
  This is also partially motivated by making it more similar to the
  other "Hello, world!" programs for consistency.
* Type-annotate the Haskell main function --- it's considered reasonably
  good style, plus we get to see the highlighting of Haskell types.
* Give the "Hello, world!" strings uniform capitalization and
  punctuation