segment: Distinguish snippets from full files.

The comment on segmenter_init() explains what this means:

If IS_SNIPPET is false, then the segmenter will parse as if it's being
given a whole file.  This means, for example, that it will interpret -
or + at the beginning of the syntax as a separator between commands
(since - or + at the beginning of a line has this meaning).

If IS_SNIPPET is true, then the segmenter will parse as if it's being
given an isolated piece of syntax.  This means that, for example, that
it will interpret - or + at the beginning of the syntax as an operator
token or (if followed by a digit) as part of a number.