Add pattern matching to `when` clause, when contributing menus

[fbricon]

• VSCode Version: 1.12.0
• OS Version: MacOS Sierra 10.12.4

In vscode-java, we'd like to be able to provide context menus for all gradle files. The current when clause is limited to language types ("when": "resourceLangId == groovy") or a single file name ("when": "resourceFilename == 'build.gradle'").

It would be helpful if there was support for glob pattern matching, as @jrieken mentioned in #3192 (comment)

[gawicks]

Hi, Any update on this?