Add RC format support to updateConfig

[Copilot]

Users need to update RC config files programmatically, but updateConfig only supports JS/TS/JSON formats.

Changes

• New utility: updateConfigRC exposes rc9's update functionality
  • Updates RC files in specified/current directory
• Auto-flattening: Nested objects automatically flatten to RC's dotted key format
• Directory creation: Ensures target directory exists before writing

Usage

import { updateConfigRC } from "c12/update";

// Update project RC config
await updateConfigRC({
  name: ".myapprc",
  dir: process.cwd(),
  onUpdate: (config) => {
    config.database = { host: "localhost", port: 5432 };
  },
});

RC files use simple key-value format with automatic nesting:

database.host="localhost"
database.port=5432

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add rc format support to updateConfig</issue_title>
<issue_description>### Describe the feature

I'd like to standardize our config files to use the rc format, but the current bottleneck is that updateConfig doesn't support rc files.
I think it wouldn't be too difficult to add stable support for rc format to updateConfig using rc9. What do you think about adding this feature? If it sounds reasonable, I'd be happy to contribute this myself.

Additional information

• Would you be willing to help implement this feature?</issue_description>

<agent_instructions>read current updateConfig and https://github.com/unjs/rc9 docs.

• Add new utility to c12/update
• Add JSDocs
• Update readme
• Add unit tests</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@pi0 Hi. You can use [rc9](https://github.com/unjs/rc9) to do this. We can also expose `updateConfigRC` utility from c12 that uses same.

• Fixes Add rc format support to updateConfig #292

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[pi0]

@copilot we don't need userRC utils

[Copilot]

Removed updateConfigUserRC and related code (function, interface, tests, and documentation) in 61015c4.