Upgrade to vscode-l10n

## Problem
Our current localization library (https://github.com/microsoft/vscode-nls) is no longer receiving updates. Instead, VSCode has introduced a new package (https://github.com/microsoft/vscode-l10n) which is its replacement

## Blockers
vscode-l10n requires a minimum vscode version of 1.73.0 but we currently [support 1.68.0](https://github.com/aws/aws-toolkit-vscode/blob/master/packages/core/package.json#L12)

Start with packages/core/package.json, checking the supported VS Code version and the current vscode-nls dependency. Compare the requirements and migration guidance for vscode-l10n, then trace the localization usages that depend on the existing package. Done means the replacement is used and the supported VS Code range is compatible.

Written by the indexing model from the issue text.