Preliminary to nvaccess/nvda#12445

Context
A first use of ngettext had been introduced in NVDA in nvaccess/nvda#11598 and nvaccess/nvda#12432.
However these changes were removed in nvaccess/nvda#12448.

Issue
As written by @michaelDCurran in nvaccess/nvda#12448:

However, it seems that our PO validation code in the translation system is not handling plural forms well, and is incorrectly classing translations as missing brace format variables, as it is incorrectly comparing the translation with the message before that one.
It then outputs the following error:
Message starting on line 7169
Original: "{startTime} to {endTime}"
Translated: "category {categories}"
Error: missing brace format interpolation, extra brace format interpolation
Expected: these brace format interpolations: {endTime}, {startTime}
Got: these brace format interpolations: {categories}

Changes in this PR
This PR adds the support of ngettext messages in the PO validation script.

… to be getting confused by it, and this was not the first and only issue. (nvaccess#12448)"

This reverts commit 76fc1f7.

… to be getting confused by it, and this was not the first and only issue. (nvaccess#12448)"

This reverts commit 76fc1f7.

This PR is the first step to reintroduce the distinction of singulare/plural forms when formatting UI strings. The project is as follow:
1. This PR introduce three strings to be able to test the feature.
2. 2023.2 beta phase should be used to validate more widely the feature with translators
3. A second PR during 2023.3 dev cycle will implement the plural forms for all UI strings containing only one value to format them.
4. A third PR should implement more complex cases of string formatted by two or more values, e.g. such as "table with x rows and y columns".

This step-by-step approach should allow to lose less work in case an issue occurs and requires to revert changes.

### Link to issue number:
First step to implement #12445.
Restoring #11598, #12432 and #12435 that were reverted in #12448.
Unblocked by nvaccess/mrconfig#97.
### Summary of the issue:
Some UI messages or strings are reported with plural form, no matter the number passed as parameter to format them, e.g. NVDA reports "list with 1 items" with "s" even if there is only one item.

### Description of user facing changes
The following strings will be reported using singular or plural form depending on the number used to format them:
* "with %s items" (used to describe the number of items in a list on the web)
* ".1f lines" (used when reporting multiple line spacing formatting in MS Word)
* ""categories {categories}" used to report categories of appointments in MS Outlook

Translators will be able to translate singular or plural forms.

### Description of development approach
* First revert #12448 to restore the first attempt that was made to introduce `ngettext`.
* Develop a custom `npgettext` function the same way as `pgettext` was introduced in NVDA since `npgettext` is not available in Python 3.7; `pgettext` and `npgettext` are available natively in Python 3.8.
* `ngettext` and `npgettext` are made accessible without importing them; `pgettext` was already added previously in builtins.
* Implement the plural form for two strings that are more commonly used than Outlook appointments category reporting:
  * "with %s items", using `ngettext`
  * "%.1f lines", using `npgettext`