feat: add flood script

[hstove]

This adds a new TS script, flood.ts. It is not added to the docker-compose setup. The contract works by:

• Using ENV var NUM_FLOODERS to determine how many random accounts to generate
• At bootup, it deploys a contract (flood.clar) and transfers STX to the "flooder" accounts
• During each loop, it makes N (TX_PER_FLOOD) transactions from each flooder. At the moment, this tx is just a semi-heavy contract call to flood.clar with chained nonces.

Run with npm run flood in the stacking dir, while the regtest-env is running.

[hstove]

Merging this - I've just tested it again and it works well!