clang-format version detection is slightly broken (and definitely so for trunk builds)

At https://github.com/google/vim-codefmt/blob/293c208/autoload/codefmt/clangformat.vim#L32-34, we have this code:

```viml
let l:version_string = matchstr(l:version_output, '\v\d+(.\d+)+')
" If no version string was matched, cached version will be an empty list.
let s:clang_format_version = map(split(l:version_string, '\.'), 'v:val + 0')
```

which is trying to find a version number (possibly "1.2.3" or "1.2" or just "1") in the output of `clang-format --version`:

```sh
$ clang-format --version
clang-format version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)
```

However, for trunk builds, the output looks something like:

```sh
clang-format --version
clang-format version mainline (4321b9f2e9842982d13234920a643e3a4657c60b)
```

(where "mainline" can be any string the configurer chooses).

However:

- `matchstr(l:version_output, '\v\d+(.\d+)+')` has `.` matching _any_ character. That probably was intended to be a literal (`\.`) instead, otherwise the pattern could just have been `\v\d.+`.
- As a result, we consider any trunk build to be a version "number" containing the numeric start of the hex string (i.e. version 4321 in the above example).
- This means that we usually consider trunk builds to have all features (good), but only if their version string starts with enough non-zero/non-hex digits (bad).

We probably should do something like:

1. Change the regexp to use `\.` instead to match a literal.
2. Consider a non-standard version to always _have_ the feature rather than not have it (at https://github.com/google/vim-codefmt/blob/293c208/autoload/codefmt/clangformat.vim#L38.

Commencez dans autoload/codefmt/clangformat.vim, au niveau de l’analyse de version autour des lignes 32-38, et comparez son comportement avec les sorties numériques et de type trunk de clang-format --version présentées dans l’issue. Vérifiez que les versions littérales comportant des points sont correctement analysées et que les versions non standard reçoivent le traitement des fonctionnalités prévu.

Rédigé par le modèle d'indexation à partir du texte de l'issue.