Terminal suggest: Request completion sequence breaks when PSReadLine is disabled, clarify whether we enable PSReadLine for screen readers

[Tyriar]

Repro:

1. Windows
2. Enable NVDA
3. Enable terminal suggest
4. Ensure shell integration is not loading PSReadLine
5. Type echo a, 🐛 es keep repeating
   

Right now the intent was for PSReadLine not to be loaded but it was being loaded by module auto loading due to this line:

vscode/src/vs/workbench/contrib/terminal/common/scripts/shellIntegration.ps1

Line 164 in 483464c

$ContinuationPrompt = (Get-PSReadLineOption).ContinuationPrompt

When we fix that, we need to either:

• always import PSReadLine explicitly before it's used, this would get 633;E to light up for screen reader users, or
• ensure we never send this request completions sequence when PSReadLine is not loaded.

Additionally we should prevent the possibility of getting into a loop like this, I'm not sure why that's happening