Jest: Coverage HTML report colors should follow `coverageThreshold` values

[nemoDreamer]

🚀 Feature Proposal

Istanbul / lcov provides a way to define min/med values to correspond to the red/yellow/green visual clues.

This is done via settings in a .lcovrc file:
https://www.systutorials.com/docs/linux/man/5-lcovrc/
https://stackoverflow.com/questions/40618939/lcov-html-report-coverage-limits

Jest allows us to define custom coverage thresholds using --coverageThreshold CLI argument/config file setting.

The coloration of the lcov HTML report should follow these values and not color a missed threshold green.

Motivation

We've set our global thresholds to 100%:

coverageThreshold: {
  global: {
    // 100%!
    branches: 100,
    functions: 100,
    lines: 100,
    // no uncovered statements!
    statements: 0,
  },
},

But (outside of the CLI) we have to painstakingly scan our coverage report to find that one 98.72% miss:

Example

Some possible solutions:

Zero-config?

We realize that Jest's coverageThreshold is more granular, allowing for global and per-glob overrides, but at least mapping the global lines/branches/functions to lcov's genhtml_hi_limit/genhtml_branch_hi_limit/genhtml_function_hi_limit would be a big improvement.

.lcovrc support?

Allow a discoverable .lcovrc (this would allow the use of _all_ lcov config options). Start discovery in package root or sibling to config.js? Or add an --lcovConfig arg alongside --config/-c?

Pitch

Since the coverageThreshold setting is a core setting, and not only supported by a custom reporter, we feel that this belongs here.

[yuliaHope]

@nemoDreamer you can use .istanbul.yml for this purpose.
For Example:

reporting:
    watermarks:
        statements: [50, 100]
        lines: [50, 100]
        functions: [50, 100]
        branches: [50, 100]

Where the firs value is top border for low coverage and the second one is bottom border for high coverage.

[SimenB]

I think we can fold this into #4103 - it's basically "configure coverage reporter".