process: factor out a helper function for running tools

[stamblerre]

cp.execFile is used throughout the codebase to run various commands. Considering we run different tools all the time, it would be useful to have a helper function to abstract this a bit. #91 is a good justification for it.

We might also want to have a separate set of helpers for running go commands specifically.

[stamblerre]

Another thing to consider is isolating CPU-intensive tools (see microsoft/vscode-go#1557). Currently, the resource intensive tools that I'm aware of are gopls and golangci-lint.

[hyangah]

Features to add as we work on this:

• cancellation (Apply timeouts on child processes #91)
• tracing of tool execution (for diagnosis)
• ensure wd and environment variables are set appropriately
  ...