Change approach to starting the Gradle Server and fix dist download progress

[badsyntax]

Fixes #512
Fixes #536

Changes

• Gradle dist progress is now reported and is cancellable
• Start server in background using node APIs. Server terminal is no longer displayed
• Server logs are displayed in the "Gradle Tasks" output channel
• Better (graceful) server restart logic (cancel all builds before gracefully killing the server with SIGTERM)
• Update README
• Due to change in how the server is started, this could be a breaking change for users who might have set ENV vars in terminal.integrated.env.* - Not sure what to do about this...

Screenshots:

Server start, and showing gradle dist download progress:

Server killed (with killall java from outside vscode):

Server exception (nullpointer) shown in the output channel:

Improved error messaging in task terminals when server exceptions occur:

TODO:

• Update README for troubleshooting instructions for ENV/PATH issues
• Add screenshot that shows what an uncaught exception looks like (with a long stacktrace)

[badsyntax]

Remove server task definition from package.json

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
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0.0% Duplication

The version of Java (1.8.0_252) you have used to run this analysis is deprecated and we will stop accepting it from October 2020. Please update to at least Java 11.
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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 2 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

The version of Java (1.8.0_262) you have used to run this analysis is deprecated and we will stop accepting it from October 2020. Please update to at least Java 11.
Read more here