builtin: add array.last_index()

[kbkpbot]

Feature request by #26247

generated C code would like this:

// V auto functions:
static int Array_int_index(Array_int a, int v) {
        int* pelem = a.data;
        for (int i = 0; i < a.len; ++i, ++pelem) {
                if (*pelem == v) {
                        return i;
                }
        }
        return -1;
}

static int Array_int_last_index(Array_int a, int v) {
        if (a.len == 0) return -1;
        static int Array_int_last_index(Array_int a, int v) {
        if (a.len == 0) return -1;
        int* pelem = (byte*)a.data + (a.len-1)*a.element_size;
        for (int i = a.len-1; i >= 0; --i, --pelem) {
                if (*pelem == v) {
                        return i;
                }
        }
        return -1;
}

[spytheman]

Excellent work.
Thank you @kbkpbot .