Create a github issue template on this repo for applying to the TSC

This is the natural follow-up from #21 and today's TSC meeting and requests creating a GitHub issue template on this TSC repository that can be used by community members to apply to the TSC.

We also discussed adding text like the following to the top of the application form to set expectations:

> The following is a list of things we will consider and value in a TSC applicant, but it’s not exhaustive. Please note that we don’t expect applicants to meet all of the following criteria, and will consider applications holistically:
> * uses of Chapel in applications
> * contributions to the Chapel community in the form of PRs (code, doc changes)
> * involvement in Chapel discussions and issues
> * organizing of, or participation in, Chapel community events (such as ChapelCon)
> * having publications, blog posts, or social media activity related to Chapel
> * providing funding to the Chapel project or other Chapel-related development efforts
> * unique perspectives, strengths, or experiences that you'd bring
> * other potential benefits to the project

@arifthpe pointed out that we should tweak the introductory statements a bit to indicate that having these things also doesn't guarantee acceptance to the TSC. To that end, I'm proposing it be updated to:

> The following is a non-exhaustive list of things we will consider and value in a TSC candidate. Please note that we don't expect applicants to meet all of the following criteria, nor does meeting any of them guarantee acceptance into the TSC. We will consider applications holistically.

Read the related issue #21 and inspect this repository for any existing GitHub issue templates. Create a TSC application template using the proposed introductory text and criteria; it is done when community members can use the template to submit an application with the stated expectations.

Written by the indexing model from the issue text.