feat(bundler): allow including custom files on debian package, fix #1428

[lucasfernog]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Configuring it on tauri.conf.json:

{
  "tauri": {
    "bundle": {
      "deb": {
        "files": {
          "/usr/lib/README.md": "../README.md", // copies a single file
          "usr/lib/assets": "../public/" // copies the entire public directory to /usr/lib/assets
        }
      }
    }
  }
}