Improve the presentation of syntax outlines

[krilnon]

When introducing new syntax for the first time, the book uses placeholder literals to show the customizable portion(s) of the syntax being introduced. Right now, these render as regular Swift code blocks that don't have a special treatment for placeholders.

rdar://64038462

[amartini51]

For example, here's the syntax outline used to illustrate the syntax of a for loop:

for <#item#> in <#collection#> {
   <#statements#>
}

The current TSPL pipeline styles them as blue bubbles, borrowing from the Xcode styling for placeholder tokens: