introduce 'deemphasize' hint on Source type

[weinand]

Some debug adapters want to deemphasize Source objects in the UI, e.g. to indicate that the real source is not available or that the source is skipped on stepping.
This request asks for add an optional deemphasize attribute to the Source type.
A front end could use this attribute by rendering the source in a dimmed way.

[weinand]

I'v added this attribute to Source:

/** An optional hint for how to present the source in the UI. A value of 'deemphasize' can be used to indicate that the source is not available or that it is skipped on stepping. */
presentationHint?: 'emphasize' | 'deemphasize';