Add "working-directory:" input

[khaeru]

This PR adds an input working-directory:, the same name as GitHub Actions uses for generic (run:) steps: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepsrun

The value is passed to the cwd option of GitHub's @actions/exec exec() method:
https://github.com/actions/toolkit/blob/d9347d4ab99fd507c0b9104b2cf79fb44fcc827d/packages/exec/src/interfaces.ts#L5-L7

This allows the user to specify in which directory codecov.sh should run.

Other information:

• I have looked at Add 'config' input to specify path to codecov.yml #70 and The yml optional input doesn't seem to be working #51 (comment) —this PR differs because it is not trying to address the deprecated/removed -y option to codecov.sh, but rather allow configuration of the directory in which the program is executed.
• The current instruction (e.g. in The yml optional input doesn't seem to be working #51) to "place codecov.yml at the root of the repository" only covers a subset of possible workflows. This is because GitHub's provided Checkout action has a path: option that controls where each repository is checked out. $GITHUB_WORKSPACE (where this action currently always executes) will be a different directory from "the root of the repository" any time path: is used; this is common when a workflow checks out two or more repositories. This PR removes this restriction.
• I am not a JS developer. There are two things I suspect may need extra work, and I'd appreciate pointers on how to address them, or please push to this branch.
  • Aside from the top-level index.js that I've modified, dist/index.js appears to have the same content embedded. There are no instructions or hints on how to generate this file.
  • The path where codecov.sh is written may need to be inside the working directory, if any: https://github.com/codecov/codecov-action/blob/master/index.js#L42

[codecov]

Codecov Report

Merging #110 (c770ad4) into master (beb5a96) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #110   +/-   ##
=======================================
  Coverage   87.50%   87.50%           
=======================================
  Files           2        2           
  Lines          16       16           
  Branches        2        2           
=======================================
  Hits           14       14           
  Misses          2        2           

Flag	Coverage Δ
unittest	87.50% <ø> (ø)

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[thomasrockhu]

@khaeru, if you resolve the conflicts, I'll take a look at merging this in.

[khaeru]

Rebased.