feat(vscode): AffineScript port of extension (TS→AS campaign step 1 / #240) (#22)

Adds editors/vscode/src/extension.affine — AffineScript port of
extension.ts, parallel artifact alongside the TS source.

Step 1 of the estate-wide TS→AffineScript migration umbrella
(hyperpolymath/standards#239); seed-port issue #240.

Build pipeline integration (compile extension.affine → out/extension.cjs,
swap package.json `main`, drop tsconfig/typescript devDeps) lands in a
follow-up PR. extension.ts remains live until that swap.

Binding coverage confirmed at this session: Vscode.affine (55 fns) +
VscodeLanguageClient.affine cover all surface the TS used
(getConfiguration, workspaceConfigGetString, newLanguageClient,
languageClientStart). Pattern mirrors affinescript dogfood at
affinescript/editors/vscode/src/extension.affine.

Behavioural notes documented inline:
  - unused `path` import omitted
  - process.env preserved by Node child_process default behaviour
    (host shim handles it)
  - synchronize.fileEvents collapsed in newLanguageClient ABI
  - top-level held-client pattern replaced by activate-only start
    (process-exit closes the server pipe)

Pre-PR ownership gate: owner=hyperpolymath, isFork=false, parent=null
(verified inline).

Author: hyperpolymath