Add optional digits to amount format

[JannickWeisshaupt]

Various UnitCode, e.g. MLT (Millilitre) imply sub-cent amounts and sub-cent amounts are generally allowed by the en16931 and Xrechnung standards.

Currently these sub-cent are omitted, which leads to seemingly false invoices, as the product of the charge amount times the quantity no longer equals the line total amount.

This PR adds 10 optional sub-cent digits to the amount-format entry for both german and english internalization files.

[bdewein]

@JannickWeisshaupt Thanks for reporting!
Restriction to two digits should be kept for amount-format in general, however, and more digits only allowed where appropriate.
Please track here: https://projekte.kosit.org/xrechnung/xrechnung-visualization/-/issues/139

[JannickWeisshaupt]

@bdewein thanks for taking this up. One question: I saw that you are going to use the at-least-two-format for specific fields instead of increasing the digits generally as I proposed. This sounds like a good idea.
However should the at-least-two-format not be
###,##0.00###############
instead of
###,##0.################# ?

[jochenkirstaetter]

Reviewing progress on GitLab - https://projekte.kosit.org/xrechnung/xrechnung-visualization/-/merge_requests/91 - this seems to be resolved and the PR should be closed, no?