Improve pluralization of terms for summaries in the product

> In every case when we say `warning(s)` or `error(s)` we _know_ if the quantity is plural or not, so we should just pluralize the noun correctly - both on this final-build summary and on the per-project summaries.

See [discussion](https://github.com/dotnet/msbuild/pull/9920#issuecomment-2018237391) in the PR.

In short, the current RESX-based localization method makes it _very_ hard to report pluralized and singular terms for values without a large amount of developer overhead. This desired error report summary is one example, and in https://github.com/dotnet/msbuild/pull/9929 @nohwnd and I were discussing the same problem - reporting singular and plural forms of successes, failures, and skipped tests leads to an _explosion_ of terms in order to support different language grammars and layout.

We need a system that doesn't impose so much burden on developers.