Add tokenBalance api endpoint

[7i7o]

Summary

This PR adds a new function to the api called tokenBalance that requires the "ACCESS_TOKENS" permissions

You can fetch the balance of a token by providing the token's id:

const tokenBalance = await window.arweaveWallet.tokenBalance("<AO_TOKEN_ID>");

It should return the token's balance in a string in the same format as userTokens returns the balances for each token

Tasks:

• Add tokenBalance api
• Add docs feat: add token balance doc docs#7
• Update arconnect types feat: add token balance type types#5

[pawanpaudel93]

tACK