PSR12.Files.ImportStatement could enforce lowercase const/function in use statements

[umherirrender]

Since #2212 the words const and function in use statements are not tokenized as T_CONST or T_FUNCTION and therefor not covered by Generic.PHP.LowerCaseKeyword.

Let PSR12.Files.ImportStatement do it instead.

[jrfnl]

@umherirrender Actually, #2212 only changed it for those keywords when used within a group use statement, but either way, I agree this would be a good additional check.

I can think of some other cases where keywords may not always be tokenized as such, but that's for another issue.

If you're interested, PHPCSExtra now contains a sniff which handles this - Universal.UseStatements.LowercaseFunctionConst. See PHPCSStandards/PHPCSExtra#58