Provide Windows compatible "npm install"

currently on windows it is failing this way:

npm install

> vscode-spring-boot-dashboard@0.1.3 postinstall C:\git\vscode-spring-boot-dashboard
> node ./node_modules/vscode/bin/install && sh -c scripts/build-java-extension.sh

Detected VS Code engine version: ^1.24.0
Found minimal version that qualifies engine range: 1.24.0
Fetching vscode.d.ts from: https://raw.githubusercontent.com/Microsoft/vscode/6a6e02cef0f2122ee1469765b704faf5d0e0d859/src/vs/vscode.d.ts
vscode.d.ts successfully installed!

: invalid optionva-extension.sh: line 2: set: -
set: usage: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
: No such file or directoryn.sh: line 5: cd: java-extension
scripts/build-java-extension.sh: line 6: ./mvnw: No such file or directory
cp: cannot stat 'target/*.jar': No such file or directory
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! vscode-spring-boot-dashboard@0.1.3 postinstall: `node ./node_modules/vscode/bin/install && sh -c scripts/build-java-extension.sh`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the vscode-spring-boot-dashboard@0.1.3 postinstall script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\Aurelien Pupier\AppData\Roaming\npm-cache\_logs\2018-11-12T16_00_14_532Z-debug.log

it will allow potentially more contributions

Start with the package postinstall command and scripts/build-java-extension.sh, then reproduce npm install on Windows using the failure output as a guide. Check the shell commands involving node_modules/vscode/bin/install, sh, cd, mvnw, and copying the JAR. Done means npm install completes successfully on Windows and the Java extension build artifacts are produced.