Fix cpptools disabled check to be case-insensitive

[appden]

The casing for C_Cpp.intelliSenseEngine was recently changed in microsoft/vscode-cpptools#9862 to be lowercase, but this extension will complain every 5 seconds unless the configuration value is strictly the capitalized "Disabled". This uses toLowerCase() to still support the legacy capitalized value just like the cpptools extension.

[sam-mccall]

Thanks!