Drop Istanbul to kill install warnings

[dead-claudia]

Description

Title's pretty self-explanatory. Also, this isn't really used much in practice.

Motivation and Context

From a local run:

$ npm ci
npm warn deprecated @humanwhocodes/config-array@0.13.0: Use @eslint/config-array instead     
d
npm warn deprecated istanbul@0.4.5: This module is no longer maintained, try this instead:
npm warn deprecated   npm i nyc
npm warn deprecated Visit https://istanbul.js.org/integrations for other alternatives.

added 239 packages, and audited 240 packages in 9s

42 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

How Has This Been Tested?

npm install after the patch:

$ npm i

up to date, audited 212 packages in 2s

42 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

Types of changes

• 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 change)

Checklist

• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change requires a documentation update, and I've opened a pull request to update them already: https://github.com/MithrilJS/docs/pulls/NNNN
• I have read https://mithril.js.org/contributing.html.