The IsEmail method doesn't comply with RFC 5321

[Lightczx]

Describe the bug

CommunityToolkit.Common.StringExtensions.IsEmail method use a regex to check if a string is a valid email,
it only allows lower-case local part.
But according to RFC5321 2.4

The local-part of a mailbox MUST BE treated as case sensitive.

Steps to reproduce

"TEST@myhost.com".IsEmail() == false

Expected behavior

"TEST@myhost.com".IsEmail() == true

Screenshots

No response

Code Platform

• UWP
• WinAppSDK / WinUI 3
• Web Assembly (WASM)
• Android
• iOS
• MacOS
• Linux / GTK

Windows Build Number

• Windows 10 1809 (Build 17763)
• Windows 10 1903 (Build 18362)
• Windows 10 1909 (Build 18363)
• Windows 10 2004 (Build 19041)
• Windows 10 20H2 (Build 19042)
• Windows 10 21H1 (Build 19043)
• Windows 10 21H2 (Build 19044)
• Windows 10 22H2 (Build 19045)
• Windows 11 21H2 (Build 22000)
• Other (specify)

Other Windows Build number

No response

App minimum and target SDK version

• Windows 10, version 1809 (Build 17763)
• Windows 10, version 1903 (Build 18362)
• Windows 10, version 1909 (Build 18363)
• Windows 10, version 2004 (Build 19041)
• Windows 10, version 2104 (Build 20348)
• Windows 11, version 22H2 (Build 22000)
• Other (specify)

Other SDK version

No response

Visual Studio Version

2022

Visual Studio Build Number

No response

Device form factor

No response

Additional context

No response

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