Add CodeLens support for resolving environment variables in string values

[BeardedFish]

Would be a helpful feature to add to the extension. Adding this will result in a developer instantly seeing what a string resolves to if it contains an environment variable.

For example, if the following config entry below existed and the value of ASSET_DOWNLOAD_RULE was deny, then at the top of this line it would show something like "${ASSET_DOWNLOAD_RULE}" resolves to "deny"

/0070 { /type "${ASSET_DOWNLOAD_RULE}" /method "GET" /url "*.assetdownload.zip/assets.zip*" }