Unify the definitions of "Markdown" behind editor extension loading

What does this MR do and why?

Resolves Live preview is driven by two competing definit... (#619238 - closed).

Drive loading and unloading of the Markdown editor extensions from the file name field alone, synced on every input event via isMarkdownFilePath. A single definition of "Markdown" now covers both the initial load and every file name edit afterwards. Monaco's language detection now only affects syntax highlighting.

Tip

Counterpart to Sync snippet Markdown live preview with mid-edi... (!252609 - merged), which takes the same approach.

Races between typing and the async load

Syncing on every keystroke means a typing burst can pass through several Markdown file names while an extension load is still in flight. The guards from Fix races between Markdown extension loading an... (!251306 - merged) keep this safe. For example:

---
title: Two Markdown names in one typing burst
config:
  fontSize: 14
---
sequenceDiagram
    autonumber
    actor User
    participant EditBlob
    participant Import as dynamic import

    Note over EditBlob: editing README
    User->>EditBlob: types → README.mkd (input event per keystroke)
    EditBlob->>+Import: start loading the Markdown extensions (promise cached)

    rect
        note right of User: race window — the load is still in flight
        User->>EditBlob: keeps typing → README.mkdn
        Note over EditBlob: still Markdown — install hits the cached promise,<br>no second load
    end

    Import-->>-EditBlob: import resolves
    Note over EditBlob: guard passes → use() once

Screen recordings

Manually verified in GDK-in-a-box:

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