Flag incorrectly constructed mailto links

## What problem would the enhancement address for VIP?

Some developers are not aware that `esc_url()` supports more than just the `http` and `https` protocols. The default list also includes ftp, ftps, mailto, news, irc, gopher, nntp, feed, and telnet as well.

of those extra ones, the most common is `mailto`, and a common mistake is to split a URL into a static `'mailto:'` and a email address variable/string escaped with something that isn't `esc_url()`.

## Describe the solution you'd like

Add a new sniff, or consider improving ProperEscapingFunction, so that we look for `'mailto:' string before an escaping function.

## What code should be reported as a violation?

```php
Email us

Email us

Email us

Email us

Email us

Email us
```

There are likely other ways to get a similar output.

## What code should *not* be reported as a violation?

```php
Email us

">Email us
```

Issue で言及されている ProperEscapingFunction sniff から始め、一覧にある違反する PHP の例と違反しない PHP の例の処理を比較してください。誤って構築された mailto リンクが報告され、esc_url() を通過した mailto 値が受け入れられれば完了です。

索引モデルが issue の本文から書いたものです。