Replace vscode-nls with @vscode/l10n

[GrahamStrickland]

vscode-nls is in maintenance mode. It was only doing the {0} placeholder substitution here — the lookup was already hand-rolled against package.nls*.json — so @vscode/l10n.t() drops straight into its place.

Call sites deliberately stay off vscode.l10n.t(). Two structural blockers:

1. debugger.ts bundles this module and runs as a standalone adapter process (vscode is external in client/build.js), where require('vscode') throws. @vscode/l10n exists for exactly that case.
2. The formatter webview localizes by computed key — formatEditor.ts:82 passes the manifest's own %key% strings through verbatim — which a literal-message API cannot express.

package.nls*.json stays regardless: 46 of its 76 keys localize the manifest. Moving runtime strings to l10n/bundle.l10n.*.json would mean two message stores and duplicated translations for no user-visible gain, and only becomes worthwhile if #12 stops the webview deriving labels from the manifest.

Also in here:

• myLocalize.ts → messageBundle.ts. Committed as a pure rename (100% similarity), so git log --follow still traces the full history.
• 10 tests for the module, which had none.
• isolatedModules set in both test tsconfigs, silencing a ts-jest warning introduced by the node16 move in Upgrade vscode-languageclient v8 → v9 (+ matching server package) #10.

Adapter bundle drops 59.0 KB → 53.6 KB. Client tests 88 → 98; server 78 unchanged. CI green on all three runners.

Bumps client to 1.4.2, server to 0.10.2.

Closes #11

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