Doesn't work when `#:lang` is set to `"scribble/manual"`

[dented42]

Using "scribble/manual" as the language for the code example doesn't seem to work. An exception related to string manipulation is raised instead.

#lang scribble/manual
@(require scribble-code-examples)
@code-examples[#:lang "scribble/manual" #:context #'here]|{@title{Thing}}|

The result is:

substring: ending index is smaller than starting index
  ending index: 22
  starting index: 35
  valid range: [0, 35]
  string: "#lang scribble/manual\n@title{Thing}"

It also does't work with "scribble/base", but "scribble/text" works as expected.

[AlexKnauth]

I started a branch to work on this here:
https://github.com/AlexKnauth/scribble-code-examples/tree/issue-13

I can fix the substring error, but the results still aren't right, they look something like:

> @title{Thing}
(part #f (list (list 'part (generated-tag))) #f (style #f (list (document-version "8.12.0.14") (latex-defaults '(collects #"scribble" #"manual-prefix.tex") '(collects #"scribble" #"manual-style.tex") '()) (html-defaults '(collects #"scribble" #"scribble-prefix.html") '(collects #"scribble" #"manual-style.css") '((collects #"scribble" #"manual-fonts.css"))) (css-style-addition '(collects #"scribble" #"manual-racket.css")) (js-style-addition '(collects #"scribble" #"manual-racket.js")))) '() '() '())
>
"\n"
> @title{Thing}
(title-decl #f '((part "Thing")) #f (style #f '()) '("Thing"))

If I can find a consistent way to include the last @title{Thing} but discard the first two things one, that might be a good thing to implement. @default-kramer had some ideas that might be relevant in #10, so I'll start looking there.