Prettier breaks XHTML documents by incorrectly lowercasing DOCTYPE
The change in https://github.com/prettier/prettier/pull/7391 caused prettier to break my XHTML documents by lowercasing the DOCTYPE, resulting in XML well-formedness errors for my use case.

DOCTYPE is case-sensitive in XML contexts. I cannot wrap it with prettier ignore comments as that would produce invalid XML.
Proposal: Add `--doctype-case` option for customizable DOCTYPE casing
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**Prettier 3.3.3**

**Input:**

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    
</body>
</html>
```

**Output:**

```html
<!doctype html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Document</title>
  </head>
  <body></body>
</html>
```

**Expected output:**

### Depends on the --doctype-case option setting:
For `--doctype-case lowercase` (can be default):
```html
<!doctype html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body></body>
</html>
```
For --doctype-case uppercase:

```html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body></body>
</html>
```
For `--doctype-case preserve`:

```html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body></body>
</html>
```

**Why?**

Proposal to add a new configuration option `--doctype-case` to allow users to choose their preferred **DOCTYPE** casing style.

- Specification compliance: The **HTML5** specification allows for **DOCTYPE** to be written in both lowercase and uppercase (case-insensitive). However, the HTML specification explicitly states: "In other words, `<!DOCTYPE html>`, case-insensitively." This suggests that uppercase is the canonical form, which some developers prefer to follow strictly. https://html.spec.whatwg.org/multipage/syntax.html#the-doctype
- Special nature of DOCTYPE: It's important to note that the **DOCTYPE i**s not an HTML tag, but rather a required **preamble**. Its unique status as a declaration rather than a tag makes it a special case in HTML documents, which some argue warrants special consideration in formatting.
- Flexibility: Different developers and teams have different preferences regarding code style. Adding this option will allow for customization of formatting to individual or team coding standards.
- Backward compatibility: The preserve option will allow maintaining existing style in projects that don't want to introduce changes in DOCTYPE casing.
- Resolving controversy: There are differing opinions in the community about the preferred **DOCTYPE** casing. Adding this option will allow reconciling different preferences without imposing a single style on all users.
- Consistency with other tools: Some tools or environments may prefer a specific **DOCTYPE** casing style. This option will facilitate Prettier's integration with such tools. e.g. the Angular team is facing problems because of this change https://github.com/angular/angular/pull/51052#issuecomment-1974933556

The proposed implementation does not violate **Prettier's** opinionated principle as it concerns a purely stylistic aspect that does not affect the functionality or semantics of HTML code.