allow passing a variable to the structural indexer step

[lemire]

PR for #2052

It allows scripts such as...

for i in {1..10}; do
    echo "Setting  SIMDJSON_STRUCTURAL_INDEXER_STEP to $i "
    cmake -B build -D SIMDJSON_STRUCTURAL_INDEXER_STEP=$i -D SIMDJSON_DEVELOPER_MODE=ON >> /dev/null
    cmake --build build --target parse  >> /dev/null
    sudo ./build/benchmark/dom/parse -s stage1 -n 3000 jsonexamples/twitter.json
    sudo ./build/benchmark/dom/parse -s stage1 -n 3000 jsonexamples/twitter.json
    sudo ./build/benchmark/dom/parse -s stage1 -n 3000 jsonexamples/twitter.json
done

[jkeiser]

This is great, thanks!