There are certain tokens that are _never_ valid, so we might as well
catch them early in the Lex... functions rather than having to handle
them in many different contexts within the parser.
Unfortunately for now when such errors occur they tend to be echoed by
more confusing errors coming from the parser, but we'll accept that for
now.
The context where a string literal was found affects what sort of escaping
it can have, so we need to distinguish these cases so that we will only
look for and handle backslash escapes in quoted strings.
This LexConfig, LexExpression and LexTemplate set of functions allow
outside callers to use the scanner in isolation, skipping the parser.
This may be useful for use-cases such as syntax highlighting, separate
parsers (such as the one in zclwrite), and so forth. Most callers should
use the parser (once implemented) though, to get a semantic AST.
A scanner "mode" decides which state it starts in, allowing us to start
in template mode for parsing top-level templates. However, currently the
only mode implemented is "normal" mode, which is the behavior we had
before.
Although this end symbol appears as just a close-brace in source, it's
worth differentiating it because the scanner must differentiate it anyway
(to recognize moving back into template-scanning mode) and it avoids the
parser from having to similarly re-recognize the difference.
On reflection, it seems easier to maintain the necessary state we need
by doing all of the scanning in a single pass, since we can then just
use local variables within the scanner function.
