[RFC]: add `fill` method to `array/fixed-endian-factory`

### Description

This RFC proposes adding support for a `fill` method to [`@stdlib/array/fixed-endian-factory`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/fixed-endian-factory).

This method is the equivalent of the [`TypedArray.prototype.fill`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/fill) method, but for fixed-endian typed arrays.

When adding support, the following tasks should be completed:

1. The `./lib/main.js` file should be updated with the added method.
2. A new test file should be created which includes unit tests testing against expected behavior.
3. Two benchmark files should be added: one measuring baseline performance and another measuring performance as the array length increases.
4. The README should be updated to include documentation for the added method.

To provide a concrete example of what a PR adding the desired method should contain, see https://github.com/stdlib-js/stdlib/commit/956a4624c788689b1bca285856b987ea3aa32eb6, which is a commit adding `forEach` support. This should provide an idea of what is expected.

For an example of specialized typed arrays having customized methods, see

- [`@stdlib/array/bool`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/bool)
- [`@stdlib/array/complex128`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/complex128)
- [`@stdlib/array/complex64`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/complex64)

While the method proposed in this RFC will likely **not** match the implementations found in the above packages, those packages should provide a conceptual idea of what is desired. Do **not** simply copy-paste the code found in those packages.

### Notes

- If you are interested in contributing a PR which addresses this RFC and are a **first-time contributor** or not yet familiar with our project conventions, please do **not** submit LLM-generated code. Please consult our [contributing guidelines](https://github.com/stdlib-js/stdlib/blob/develop/CONTRIBUTING.md) and the associated [development guide](https://github.com/stdlib-js/stdlib/blob/develop/docs/development.md). Failure to respect project conventions will result in your PR being rejected without review. Thank you for understanding!

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `[RFC]:`.

Start with lib/main.js in @stdlib/array/fixed-endian-factory and compare the referenced forEach commit and specialized typed-array packages. Add the method, a unit-test file, two benchmark files, and README documentation covering the expected TypedArray.prototype.fill behavior. Done means the tests, benchmarks, and documentation are included.

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