Enable Base Mainnet Direct Mint (#841)

## Description

Remove TODO comments and uncomment code for Base mainnet deployments in
L2 Bitcoin Depositor and L2 TBTC Token classes. This includes:
- Importing Base mainnet deployment artifacts
- Adding Base chain case in deployment selection logic
- Preparing for full Base mainnet support

## Type of change

- [x]  💅 New Feature (Breaking/Non-breaking Change)
- [ ] 🐛 Bug fix (Non-breaking Change: Fixes an issue)
- [ ] 🛠️ Chore (Non-breaking Change: Doc updates, pkg upgrades, typos,
etc..

## Checks
- [x] Lint and format
- [x] Docs
- [x] Test and build

cross-chain/base/deployments/mainnet/.chainId

@@ -0,0 +1 @@
+1