Add functional and acceptance test harnesses and spl2 tests and bugfixes

[fantavlik]

Added testing

Most of the Splunk Extension code depends on the vscode package which requires a running instance of vscode to work. This PR adds testing harnesses using @vscode/test-electron using these resources as guidance:

• https://github.com/microsoft/vscode-extension-samples/tree/main/helloworld-test-sample
• https://code.visualstudio.com/api/working-with-extensions/testing-extension#custom-setup-with-vscodetestelectron
• https://vscode.rocks/testing/#end-to-end-testing

Functional tests: these import the code directly so that individual functions can be tested similar to unit tests but with a running instance of vscode Acceptance tests: these actually install the packaged .vsix version of the Extension into running VSCode instance in the testing environment. Unfortunately I wasn't able to get a meaningful test working yet with this but it was quite a lot of work just getting the harness working across platforms so I thought it would still be useful to work off of moving forward.

Bugfixes

• After adding functional tests this run actually uncovered a Windows Java installation bug
• Fixed a Linux bug that could apply to any OS specifying a Java version of the format openjdk version "17.0.8.1" or similar
• Fixed an issue with hardcoded java.exe being checked for Linux/Mac
• Fixed an issue with installation continuing/looping due to functions not being returned after resolve()
• Improved error messaging for common unauthenticated and endpoint-not-found scenarios

Bump LSP

• Bumped the SPL2 language server to the latest version 2.0.375