[Cashtab] Add function to filter out unspendable coinbase utxos from utxo set

Summary:
Coinbase utxos are unspendable until they have 100 confirmations.

Add a function ignoreUnspendableUtxos with unit tests.

This function is not yet implemented. Will need to add chaintipBlockheight to wallet context before implementing.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15179