Add stalebot

[GregTheGreek]

Description

This PR introduces stalebot to aid issue management. It introduces no changes to the codebase.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build-all and tested the resulting files from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.

[coveralls]

Coverage remained the same at 90.166% when pulling 74b4332 on greg/add-stale-bot into 97598a6 on 1.x.

[cgewecke]

LGTM

[ryanio]

I noticed on github.com/apps/stale it says: After installing the integration, create ```.github/stale.yml``` in the default branch to enable it:. The file in this PR is .stale.yml so it might need to be renamed?