test: 加一个 PowerShell 语法检查脚本

本项目大部分 PowerShell 是在 Mac 上写的，那里没有解析器。「看起来没问题」
不算检查 —— 一个多余字符就能让 Windows 安装器完全跑不起来，而这件事只有
在 Windows 上才能发现。

用法（在仓库根目录）：
    powershell -ExecutionPolicy Bypass -File portable\advanced\Check-PowerShell.ps1

退出码等于出错文件数，以后也能接进 CI。

portable/advanced/Check-PowerShell.ps1

@@ -0,0 +1,52 @@
+# Parses every .ps1 in the repository and reports syntax errors.
+#
+# Why this exists: most of this project's PowerShell was written on a Mac, where
+# no parser is available. "It looks right" is not a check — a single stray
+# character stops the Windows installer from running at all, and there is no way
+# to find that out except on Windows.
+#
+# Run from the repository root:
+#     powershell -ExecutionPolicy Bypass -File portable\advanced\Check-PowerShell.ps1
+#
+# Exit code is the number of files with errors, so CI can use it too.
+
+$ErrorActionPreference = 'Stop'
+$root = Split-Path -Parent (Split-Path -Parent $PSScriptRoot)
+$failed = 0
+$checked = 0
+
+Write-Host ""
+Write-Host "  Parsing every .ps1 under $root" -ForegroundColor Cyan
+Write-Host ""
+
+Get-ChildItem -Path $root -Recurse -Filter *.ps1 -File |
+    Where-Object { $_.FullName -notmatch '\\node_modules\\|\\app\\|\\\.git\\' } |
+    Sort-Object FullName |
+    ForEach-Object {
+        $checked++
+        $errors = $null
+        $relative = $_.FullName.Substring($root.Length).TrimStart('\')
+        [void][System.Management.Automation.Language.Parser]::ParseFile($_.FullName, [ref]$null, [ref]$errors)
+
+        if ($errors -and $errors.Count -gt 0) {
+            $failed++
+            Write-Host ("  FAIL  " + $relative) -ForegroundColor Red
+            foreach ($e in $errors) {
+                $line = $e.Extent.StartLineNumber
+                Write-Host ("        line " + $line + ": " + $e.Message) -ForegroundColor Red
+            }
+        } else {
+            Write-Host ("  ok    " + $relative) -ForegroundColor Green
+        }
+    }
+
+Write-Host ""
+if ($failed -eq 0) {
+    Write-Host "  $checked file(s) parsed, no syntax errors." -ForegroundColor Green
+} else {
+    Write-Host "  $failed of $checked file(s) have syntax errors." -ForegroundColor Red
+    Write-Host "  Send the red lines above to whoever wrote them." -ForegroundColor Yellow
+}
+Write-Host ""
+
+exit $failed