ci: fix typos allowlist and install-script syntax check

Allowlist 'iterm' (iTerm2 identifier) for the whole-repo spell check,
and initialize the pwsh error variable in the install-script smoke so
ParseFile's [ref] argument is valid.

Author: elkaix

.github/workflows/install-smoke.yml

@@ -36,4 +36,4 @@ jobs:
 
       - name: Check PowerShell syntax
         run: |
-          pwsh -NoProfile -Command '[void][System.Management.Automation.Language.Parser]::ParseFile("apps/pythinker-web/public/install.ps1", [ref]$null, [ref]$errs); if ($errs.Count) { $errs; exit 1 }'
+          pwsh -NoProfile -Command '$errs = $null; [void][System.Management.Automation.Language.Parser]::ParseFile("apps/pythinker-web/public/install.ps1", [ref]$null, [ref]$errs); if ($errs.Count) { $errs; exit 1 }'

_typos.toml

@@ -25,3 +25,4 @@ nd = "nd"                     # ndJsonStream, `Nd` cron interval token
 dows = "dows"                 # formatDows — days-of-week (cron)
 fo = "fo"                     # `/FO` flag of Windows schtasks
 pn = "pn"                     # "PNGs" tokenized as PN by the checker
+iterm = "iterm"               # iTerm2 terminal app identifier