grammar: introduce new grammar package

This package provides a way to convert JSON schemas to equivalent EBNF.
It is intended to be a replacement to llama.cpp's schema_to_grammar.

This is still an early version and does not yet support all JSON schema
features. The to-do list includes:

- minumum/maximum constraints on integer types
- minLength/maxLength constraints on string types
- defs and refs

grammar/internal/diff/testdata/eof1.txt

@@ -0,0 +1,18 @@
+-- old --
+a
+b
+c
+-- new --
+a
+b
+c^D
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,3 +1,3 @@
+ a
+ b
+-c
++c
+\ No newline at end of file