Add `#` as a completion trigger character

[mjbvz]

TypeScript Version: 3.8.0-beta

Search Terms:

• CompletionsTriggerCharacter
• completionInfo
• suggestions
• completions

Problem
To support private fields, VS Code now triggers suggestions when you type # (microsoft/vscode#88816) However TS Server does not yet recognize # as a valid completion trigger character.

I suggest that we add # as a valid TS completion character. This trigger character could also be used to filter down the list of items we may need to return (if completions are triggered with #, we likely only need to check private properties)

/cc @DanielRosenwasser