Feature Request: Support for Non-Conventional Commits

# Problem statement

When integrating legacy codebases or older branches into projects using release-please, commits that don't follow the conventional commit format are silently ignored. This means:
- No version bumps occur despite code changes
- Legacy commits don't appear in changelogs
- Manual intervention is required to force releases
- Migration from non-conventional to conventional commits is difficult

This is particularly problematic when:
- Merging long-lived feature branches with pre-conventional commits
- Migrating legacy projects to release-please
- Working with teams transitioning to conventional commits
- Incorporating external contributions that don't follow conventions

# Proposed Solution

PR: https://github.com/googleapis/release-please/pull/2624

Add an optional configuration setting to add any number of mappings `prefix` -> `conventional commit prefix` that allows non-conventional commits or non-accounted for prefixes (like emojis) to be treated as a specified commit type.

Example Configuration

```
{
"extra-prefix-mapping": {
"🐛": "fix",
"change": "fix",
"": "chore"
}
}
```

# Behavior

When not configured (default): Current behavior.

When configured: new mappings `prefix` -> `conventional commits prefix` are taken into account.