Useful when you need to check for more than one code point, some of
which are multiple bytes. (In which case StringView::contains() will
return wrong results.)

The previous code to determine the SourceDocument's lines was too naive:
the source text can contain other newline characters and sequences, and
the HTML/CSS/JS syntax highlighters would take those into account when
determining what line a token is on. This disagreement would cause
incorrect highlighting, or even crashes, if the source didn't solely use
`\n` for its newlines.

In order to have everyone agree on what a line is, this patch first
processes the source to replace all newlines with `\n`. The need to
copy the source like this is unfortunate, but viewing the source is a
rare enough action that this should not cause any noticeable
performance problems.

As the callers have a String, and we want a String, this also changes
the function parameters to keep the source as a String instead of
converting it to StringView and back.

Fixes LadybirdBrowser#3169