Rule request: Avoid using the Assignment by Addition Operator (`+=`) to build a string

Summary of the new feature

Strings are immutable. Each addition to the string actually creates a new string big enough to hold the contents of both the left and right operands, then copies the elements of both operands into the new string. For small strings, this overhead may not matter. For large strings, this can affect performance and memory consumption.
This will create more awareness of the possible performance impact of using the Assignment by Addition Operator (+= ), see also:

• Avoid the increase assignment operator (+=) for building strings
• PowerShell scripting performance considerations / String addition

Proposed technical implementation details (optional)

See (PowerShell based) prototype AvoidPlusEqualsToBuildStrings at: https://github.com/iRon7/PSRules

What is the latest version of PSScriptAnalyzer at the point of writing

1.21.0

首先查看链接的 AvoidPlusEqualsToBuildStrings 原型，以及 issue 中关于 PowerShell 字符串加法的引用。然后检查 PSScriptAnalyzer 现有的规则模式和测试。当某条规则能够识别使用 += 构建字符串的情况，并按照项目的分析器约定一致地报告该情况时，即视为完成。