deno-version-file: .dvmrc fails with dvm's key=value format

## Problem

`setup-deno` supports reading `.dvmrc` via `deno-version-file` (added in #32), but it expects a bare version number like `2.6.9`. However, dvm (https://github.com/justjavac/dvm) writes `.dvmrc` in `key=value` format:

```
deno_version=2.6.9
```

This causes `setup-deno` to fail with:

```
The passed version range is not valid.
```

## Expected behavior

`setup-deno` should parse the `deno_version=` prefix when reading `.dvmrc`, since that's the format dvm produces (via `dvm use --write-local`).

## Workaround

Extract the version manually before calling `setup-deno`:

```yaml
- id: deno-version
run: echo "version=$(grep -oP '(?<=deno_version=).+' .dvmrc)" >> "$GITHUB_OUTPUT"
- uses: denoland/setup-deno@v2
with:
deno-version: ${{ steps.deno-version.outputs.version }}
```