Remove generated parsers' dependency on the runtime library

Generated parsers no longer export a parser constructor function.
They now export an opaque Language object which can be set on
Documents directly. This way, the logic for constructing parsers
lives entirely in the runtime. The Languages are just structs which
have no load-time dependency on the runtime

src/runtime/lexer.h

@@ -0,0 +1,16 @@
+#ifndef RUNTIME_LEXER_H_
+#define RUNTIME_LEXER_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "tree_sitter/parser.h"
+
+TSLexer ts_lexer_make();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RUNTIME_LEXER_H_