Updates foreign currencies support

[armonge]

Previously

2022-08-03 * "Charlie" "Foreign food"
  Assets:EUR:N26:Checking              0.574997419221637245793331269 EUR
  Expenses:Financial:ForeignExchange  -0.574997419221637245793331269 EUR
  Assets:EUR:N26:Checking             -9.425002580778362754206668731 EUR {0.9687 CHF}

2022-08-04 * "Mustermann GmbH" "MasterCard Payment"
  Assets:EUR:N26:Checking  -12.21 EUR {1.0 EUR}

Now

2022-08-03 * "Charlie" "Foreign food"
  Expenses:Financial:ForeignExchange       9.13 CHF @ 0.9687 EUR
  Assets:EUR:N26:Checking             -8.844231 EUR

2022-08-04 * "Mustermann GmbH" "MasterCard Payment"
  Assets:EUR:N26:Checking  -12.21 EUR