host: Update Monaco modifier

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This removes an unnecessary conditional and extracts a setup function. It was originally in #2399.

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Host Test Results

  1 files  ± 0    1 suites  ±0   30m 16s ⏱️ + 2m 20s
921 tests +50  915 ✅ +51  6 💤 ±0  0 ❌ ±0 
928 runs  +51  922 ✅ +53  6 💤 ±0  0 ❌  - 1 

Results for commit 92719b7. ± Comparison against base commit a87b83c.

This pull request removes 1 and adds 51 tests. Note that renamed tests count towards both.

Chrome ‑ error

Chrome ‑ Acceptance | Freestyle: smoke check
Chrome ‑ Acceptance | code submode | editor tests > when the user lacks write permissions: the editor is read-only
Chrome ‑ Acceptance | code submode | editor tests: allows fixing broken cards
Chrome ‑ Acceptance | code submode | editor tests: card definition change made in monaco editor is auto-saved
Chrome ‑ Acceptance | code submode | editor tests: card instance JSON displayed in monaco editor
Chrome ‑ Acceptance | code submode | editor tests: card instance change made in card editor is auto-saved
Chrome ‑ Acceptance | code submode | editor tests: card instance change made in monaco editor is auto-saved
Chrome ‑ Acceptance | code submode | editor tests: invalid JSON card instance change made in monaco editor is NOT auto-saved
Chrome ‑ Acceptance | code submode | editor tests: non-card instance change made in monaco editor is auto-saved
Chrome ‑ Acceptance | code submode | editor tests: unsaved changes made in card editor are saved when switching out of code submode
…