Allow to overwrite `whatBump` function from presets

[jaklan]

Hi, currently we are able to overwrite parserOpts and writerOpts:

release-notes-generator/lib/load-changelog-config.js

Lines 35 to 39 in 354354b

	return {
	parserOpts: { ...loadedConfig.parser, ...parserOpts },
	writerOpts: { ...loadedConfig.writer, ...writerOpts },
	};
	};

but we miss a capability to overwrite whatBump() function used by presets, e.g. conventional-changelog-conventionalcommits:

export default async function createPreset (config) {
  return {
    commits: {
      ignore: config?.ignoreCommits,
      merges: false
    },
    parser: createParserOpts(config),
    writer: await createWriterOpts(config),
    whatBump: createWhatBump(config)
  }
}

In case of this specific preset, whatBump() impacts the generated release notes because of a) commit object modification (which is a preset issue itself) b) hardcoded breakingHeaderPattern here:

export const breakingHeaderPattern = /^(\w*)(?:\((.*)\))?!: (.*)$/

which ignores the breakingHeaderPattern value passed to parserOpts (which, I guess, makes sense as whatBump() is a separate function). If we could overwrite a function - we would be able to align parser and whatBump() logic on our end.