feat: add sepby1 combinator

`sepby1` is a common reusable combinator in Haskell Parsec.

This adds `mpc_sepby1(mpc_fold_t f, mpc_parser_t *sep, mpc_parser_t *a)` according to Haskell's implementation:

https://hackage.haskell.org/package/parsec-3.1.16.1/docs/src/Text.Parsec.Combinator.html#sepBy1

Reuses existing `mpc_and`, `mpc_many`, and `mpcf_snd_free`.

README.md

@@ -356,6 +356,14 @@ Runs `a` one or more times until it fails. Results are combined with fold functi
 
 * * *
 
+```c
+mpc_parser_t *mpc_sepby1(mpc_fold_t f, mpc_parser_t *sep, mpc_parser_t *a);
+```
+
+Runs `a` one or more times, separated by `sep`. Results are combined with fold function `f`.
+
+* * *
+
 ```c
 mpc_parser_t *mpc_count(int n, mpc_fold_t f, mpc_parser_t *a, mpc_dtor_t da);
 ```