Provide "prettification" extension method FirstToUpper for string

### Overview

Hello, here is another entry of my proposals for functionalities to import from my current CommonNET project to the Microsoft Community Toolkit, I hope you find it useful (and if not, nothing happens). Thanks in any case :)

In case you need to prettify a string for some reason, and you can't be sure about how the string currently will be received, this methods can help to format them appropriately:

- **FirstToUpper**

Since it is a simple implementation, mainly the proposal is to simplify its use as an extension method by extending the current SDK.

### API breakdown

```csharp

public static class StringExtensions
{
// [...] Current methods

public static string FirstToUpper(this string str, thenToLower = true)
{
//Logic
}

```

### Usage example

```csharp

string text = "hello world!";

Console.WriteLine(text.FirstToUpper());

//Hello world!

```

### Breaking change?

No

### Alternatives

-

### Additional context

Any other related methods or considerations? different API definition or naming?

This is one of my proposals of a series of implementations that I made during time in a common project used for the projects of my OpenSource organization, personal projects and projects of the company in which I work.

Currently, I would prefer to include what is practical in a repository with greater scope and practicality, and eliminate what is not necessary or could be improved, leaving the library in a state of maintenance without new developments.

Please refer to [this](https://github.com/ChustaSoft/commonnet) project

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