Support for nested functions (GNU C)

https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html

Example:

```C
#include

int main(void) {
void test(void) {
printf("A\n");
}
test();
return 0;
}
```

```shell
$ claudes-c-compiler/target/release/ccc-x86 a.c -o a
a.c:5:18: error: expected ';' after declaration before '{'
void test(void) {
^
fix-it hint: insert ';'
ccc: error: a.c: 1 parse error(s)
```