Milestone 1.2: Type definitions for midi, skulpt, math expressions

[nishantwrp]

Overview

1. This PR fixes or fixes part of #[fill_in_number_here]. #6351
2. This PR does the following: Type definitions for midi, skulpt, math expressions

Essential Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The linter/Karma presubmit checks have passed locally on your machine.
• "Allow edits from maintainers" is checked. (See here for instructions on how to enable it.)
  • This lets reviewers restart your CircleCI tests for you.
• The PR is made from a branch that's not called "develop".

PR Pointers

• Oppiabot will notify you when you don't add a PR_CHANGELOG label. If you are unable to do so, please @-mention a code owner (who will be in the Reviewers list), or ask on Gitter.
• For what code owners will expect, see the Code Owner's wiki page.
• Make sure your PR follows conventions in the style guide, otherwise this will lead to review delays
• Never force push. If you do, your PR will be closed.

[oppiabot]

Hi, @nishantwrp. This pull request does not have a "CHANGELOG: ..." label as mentioned in the PR checkbox list. Please add this label. PRs without this label will not be merged. If you are unsure of which label to add, please ask the reviewers for guidance. Thanks!

[oppiabot]

Assigning @vojtechjelinek for the first-pass review of this pull request. Thanks!

[oppiabot]

Assigning @kevinlee12 for the first-pass review of this pull request. Thanks!

[vojtechjelinek]

LGTM!

[ankita240796]

LGTM, Thanks @nishantwrp!