Support vscode-languageclient@8.1.0

[testforstephen]

Starting from LanguageClient@8.0.1, the BaseLanguageClient.start() has been changed to return a promise instead of Dispose, and onReady() is removed, they're breaking changes to vscode-java.

https://github.com/microsoft/vscode-languageserver-node/blob/release/client/8.0.1/client/src/common/client.ts#L933

We need to make changes to the client code in vscode-java to support the new behavior.

vscode-java/src/standardLanguageClient.ts

Line 105 in 97b29fa

this.languageClient.onReady().then(() => {

[rgrunber]

Just for reference, we're likely going to need to wait on Theia to support things like TypeHierarchyItem. Currently if we run the extension there, it will fail in the vscode-langaugeclient initialization code. See #2377 .

We can just update that PR when ready.