Provide an easy way to update and build a checkout of the extension

I.e. either provide a gulp command or a shell & batch scripts that:

1. git pull --prune (and potentially git checkout -- yarn.lock if it’s been modified; doing it blindly touches the file and causes yarn to fully run again)
2. yarn install
3. node node_modules/typescript/bin/tsc -p .

I already do these steps at startup on my machine and it works great, but it would be easier to maintain it with the code and get more people using to help test master.

[brettcannon]

Here is my current thinking on a Python script to handle this:

• pvsc-master.py init <code | code-insiders: Clones the repo into the appropriate location for the specified version of VS Code and then running a subset of the steps below …
• pvsc-master.py update: Checks the appropriate places for both code and code-insiders and then does the following:
  1. Delete languageServer
  2. git pull origin master
  3. npm install --no-save
  4. node .\node_modules\typescript\bin\tsc -p .
  5. Delete pythonFiles\experimental\ptvsd (unless extra logic is provided to verify if an upgrade is needed, which could include pip just doing the right thing in the next step …)
  6. -m pip install --target .\pythonFiles\experimental\ptvsd

[brettcannon]

Added the pvsc-dev-ext.py script.