Option to alter the default save location of tempRunnerFiles

(Dupe of #652, but that issue has nothing in it)

**Is your feature request related to a problem? Please describe.**
When I run a file that I haven't saved (because it's just a notepad of sorts), the `tempCodeRunnerFile.lang` is defaulted to the root of the first workspace. This is awkward because I don't want to commit this file, and I also don't really want to `.gitignore` it either, because that's an opinion.

**Describe the solution you'd like**
I'd like my temp file to be saved to a location of my choice (and 1 up, using a filename pattern of my choice). This shouldn't be synced because it's platform dependant (ie `/tmp/` won't be found on a Windows system, and `%temp%` won't be found on a *nix system).

Start by locating where tempCodeRunnerFile.lang is created and how extension settings are defined. Clarify the configurable location, parent-directory behavior, and filename pattern for unsaved files, then verify that platform-dependent paths are handled without requiring synchronized settings.

Written by the indexing model from the issue text.