[clang-tidy] Add `modernize-use-elements-view` check for `std::views::keys` / `std::views::values`

A modernization check for loops that only use one component of an associative container element would be useful in C++20 and later. In many codebases, loops still iterate key-value pairs even when only the key or only the value is actually needed.

Typical examples are:

```cpp
for (const auto &[key, value] : m) {
use(key);
}
```

or:

```cpp
for (auto &&entry : m) {
use(entry.second);
}
```

when the loop body is really interested in just one projection of the range.

In C++20, the intent can often be expressed more directly as:

```cpp
for (const auto &key : m | std::views::keys) {
use(key);
}
```

or:

```cpp
for (auto &&value : m | std::views::values) {
use(value);
}
```

This makes the loop read in terms of what it actually consumes, rather than exposing the pair structure when that structure is incidental.

Start by surveying existing clang-tidy modernize checks and their tests, then trace how checks recognize range-based loops and apply C++20 transformations. Define the supported key-only and value-only cases, including structured bindings and entry access, and add coverage showing that qualifying loops become std::views::keys or std::views::values while other loops remain unchanged.

Written by the indexing model from the issue text.