Clarify support around multiple wallets. Remove ambiguous "vault" terminology.

[mitchmindtree]

Currently, there's some ambiguity around forc wallet's support for multiple wallets.

The default "vault" directory is .fuel/wallets.

However, in practice, all uses of this directory assume a single wallet with a single set of accounts. E.g. the default .wallet and .accounts files are dumped directly in .fuel/wallets.

I propose the following:

1. For now, clarify that in order to use anything other than the default wallet, a full path to the custom wallet must be specified.
2. Change the default wallet path to .fuel/wallets/default in anticipation of potentially supporting multiple wallets (e.g. by name) under .fuel/wallets in the future.
3. Remove the "vault" terminology from the UI and code in favour of "wallet" when referring to the .wallet file. Currently, the two terms are used interchangeably throughout the code-base and help text and it can be confusing until realizing that there's no practical distinction.