Fix multipath RegEx

[rekmarks]

This updates the multipath validation to allow for a broader range of sub-paths, and adds unit tests for the input validation. Paths starting with bip32 segments will still require a parentKey to be specified, and paths starting with bip39 will still require that no parentKey is specified. Paths longer than 5 bip32 segments will not be accepted, because BIP-44 only defines paths of length 5, excluding the seed.

Specifically, we change the quantifier for intermediate bip32 path segments of the multipath RegEx from {3,4} to {0,4}. In conjunction with the other RegEx validators, this means that the following multipaths will now be accepted:

bip32:44'/bip32:60'/bip32:0'/bip32:0/bip32:0
bip32:44'/bip32:60'/bip32:0'/bip32:0
bip32:44'/bip32:60'/bip32:0'
bip32:44'/bip32:60'
bip32:44'

bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'/bip32:0'/bip32:0/bip32:0
bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'/bip32:0'/bip32:0
bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'/bip32:0'
bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'
bip39:a b c d e f g h i j k l/bip32:44'
bip39:a b c d e f g h i j k l

Prior to this PR, only the following multipaths were accepted:

bip32:44'/bip32:60'/bip32:0'/bip32:0/bip32:0
bip32:44'/bip32:60'/bip32:0'/bip32:0
bip32:44'

bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'/bip32:0'/bip32:0/bip32:0
bip39:a b c d e f g h i j k l/bip32:44'/bip32:60'/bip32:0'/bip32:0
bip39:a b c d e f g h i j k l