Improve eslint

[corasaurus-hex]

What has Changed?

This PR is an overhaul of of linting for the project while temporarily keeping it compatible with the current code. The goal is to then turn rules on a few at a time and contribute them as subsequent PRs.

For the reviewer, the general things this PR does are:

1. Removes tslint, which was deprecated in 2019.
2. Upgrades the typescript package.
3. Upgrades some eslint packages.
4. Removes some other eslint packages.
5. Adds updated configurations for eslint and typescript.
6. Adds eslint as a step in in CI.
7. Changes cp commands in the CI config to use arguments that should work cross-platform.

Fixes #1525

My Calva PR Checklist

I have:

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the PR base branch, so that it is not published. (Sorry for the nagging.)
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• Added to or updated docs in this branch, if appropriate
• Tests
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
  • Tested the VSIX built from the PR (so, after you've submitted the PR). You'll find the artifacts by clicking Show all checks in the CI section of the PR page, and then Details on the ci/circleci: build test.
• Referenced the issue I am fixing/addressing in a commit message for the pull request.
  • If I am fixing the issue, I have used GitHub's fixes/closes syntax
  • [~] If I am fixing just part of the issue, I have just referenced it w/o any of the "fixes” keywords.
• Created the issue I am fixing/addressing, if it was not present.
• Formatted all JavaScript and TypeScript code that was changed. (use the prettier extension or run npm run prettier-format)
• Confirmed that there are no linter warnings or errors (use the eslint extension or run npm run eslint).

Ping @PEZ, @bpringe

[corasaurus-hex]

@PEZ and @bpringe : this is ready for review when you get a chance 😎

[PEZ]

We should fix so that we don't need that newline in the changelog, but anyway. =)

[PEZ]

Thanks!