Allow disabling specific diagnostics

[phil-blain]

Is your feature request related to a problem? Please describe.
In my projects, the diagnostic "Variable $name masks variable in parent scope" is displayed for nearly all files since it is a common occurrence in our code to use the same variable names in called subroutines. I would like to be able to disable this specific diagnostic.

Describe the solution you'd like
I'd like all diagnostics to be individually disable-able.

Describe alternatives you've considered
Disabling all diagnostics(!)

Additional context
Maybe the existing disable_diagnostics could be turned from a boolean to a list of individual diagnostics ?