support LAMBDA_EXPRESSION in completion

[Eskibear]

Originally reported in redhat-developer/vscode-java#2253 (comment)

CompletionProp1osal.LAMBDA_EXPRESSION has been introduced in jdt.core v3.28, but we failed to construct corresponding completionItem. It goes into below code branch.

https://github.com/eclipse/eclipse.jdt.ls/blob/7ffff9d6b1c95f06261eac998210861cd5f63286/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/contentassist/CompletionProposalDescriptionProvider.java#L686-L688

[Eskibear]

In eclipse, it completes for something like ${1:t} -> ${0}:

lambda.mp4

[0dinD]

I like the way it's done in Eclipse, snippets with tabstops are very convenient.

[Eskibear]

A demo here, i'll create corresponding PR later.

lambda-vscode.mp4