Automate build and release process

[mark-wiemer]

See westes/flex#480 as mentioned in #152

Run a single command from local machine to automatically do all of the following:

1. Publish new tag to GitHub
2. Publish new release to all platforms
   • GitHub (with description from Changelog)
   • Open VSX
   • Visual Studio Marketplace

The current process is annoyingly complicated, see Development.md.

[mark-wiemer]

Continuous Integration

[mark-wiemer]

https://github.com/mark-wiemer/vscode-helloworld has promising work here, esp. mark-wiemer/hello-vscode-extension#38

[mark-wiemer]

See also #152