Add helper to detect length one string terms (#2654)

Add helper to detect length one string terms (#2654)

This commit introduces a helper function to detect string terms that
have exactly/at most length one. This is useful for a lot of rewrites
because strings of at most length one are guaranteed to not overlap
multiple components in a concatenation, which allows for more aggressive
rewriting.

This commit has been tested with --sygus-rr-synth-check for >1h on the
string term grammar.