support user-provided timestamp in custom log mode

currently the timestamp in custom log mode(fclog) is provided inner by calling `getLocalTime()`, but sometimes we should display logs with user-provided timestamps, and currently there is no way to provide the timestamp.

https://github.com/rennzhang/codemirror-editor-vue3/blob/0dfc00ea2726311aef11a2c084e79d6c7e9e640c/packages/src/components/presetMode/log/utils.ts#L141