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

[nikicat]

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:

- 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 }}