refactor: unify UI strings in central place via vscode-l10n API

[formulahendry]

This PR aims to unify UI strings into central place. In addition, by leveraging vscode-l10n API, it also supports localization in the future if needed.

Notes:

• All static strings in package.json are put into package.nls.json
• All strings in TS code are put intosrc\uiStrings.ts

References:

• https://github.com/microsoft/vscode-l10n
• https://www.eliostruyf.com/localization-visual-studio-code-extensions/

[wenytang-ms]

you may run npm install locally to install husky so the Eslint tool could help to fix the lint error.