shellIntegration.ps1: escape all non-alnum values in serialization

Instead of only escaping '\', '␤', and ';', all non-alphanumeric values
are escaped. Backslashes are doubld and other characters are encoded as
hex.

The previous version would pass invalid characters through unescaped,
including OSC sequences. Although that no longer is allowed, this
iteration does not yet enforce  particular (e.g. utf-8) encoding and
will still misbehave on multibyte sequences.

Author: rwe

src/vs/workbench/contrib/terminal/browser/media/shellIntegration.ps1

@@ -18,7 +18,17 @@ $Global:__VSCodeOriginalPrompt = $function:Prompt
 $Global:__LastHistoryId = -1
 
 function Global:__VSCode-Escape-Value([string]$value) {
-	$value.Replace("\", "\\").Replace("`n", "\x0a").Replace(";", "\x3b")
+	# NOTE: In PowerShell v6.1+, this can be written `$value -replace '…', { … }` instead of `[regex]::Replace`.
+	# Replace any non-alphanumeric characters.
+	[regex]::Replace($value, '[^a-zA-Z0-9]', { param($match)
+		# Backslashes must be doubled.
+		if ($match.Value -eq '\') {
+			'\\'
+		} else {
+			# Any other character is encoded as hex.
+			'\x{0:x2}' -f [int][char]$match.Value
+		}
+	})
 }
 
 function Global:Prompt() {