Prepare extension for localization

[philliphoff]

Prepares the extension for eventual localization:

• Adds the localization dependencies (vscode-nls)
• Updates the codebase to use the localize() function for strings and package.json localization mechanisms
• Integrates localization into the build pipeline (using vscode-nls-dev)
• Switches to Gulp as the underlying build process (to accommodate the localization tasks)
• Updates the CI pipeline accordingly

Resolves #14.

[bwateratmsft]

I think VSCODE_NLS_CONFIG gets automatically consumed by nls.config()? Should double check.

[philliphoff]

Yeah, the comment was related to other extensions which seem to pass process.env.VSCODE_NLS_CONFIG directly to the config() method, which seems redundant given that vscode-nls appears to handle that environment variable internally. Personally, I like the ability to configure just our extension as having a specific locale (e.g. pseudo), without having the entire product use that locale.

Until we actually have translations for another language I'm not 100% sure the pipeline works from start to end-packaged-VSIX. That said, it's likely to require, at most, a couple of minor tweaks (which was the point of this PR).

Another question will be how the localization pipeline integrates the with an upcoming bundling change (to use webpack).

[bwateratmsft]

Some comments but looks good!