Add an identifier to work done progress messages

[plemarquand]

Description

Work done progress messages ($/progress) contain a message and a title, but there is no id to match begin/end events up together. It is possible to use the title, but if developers want to check for specific types of work progress they'll have to key off the title and if that title ever changes in future then their code will break.

It would be nice to add an id field to these progress messages that is considered immutable. One catch is there is no id field in the protocol definition so this would be out of spec.

[ahoppen]

Synced to Apple’s issue tracker as rdar://129678779

[ahoppen]

Adding a consistent prefix to the token of the work done progress here: #1493

For crashed sourcekitd, this will be sourcekitd-crashed.