Refactor building and testing

[ssbarnea]

Summary

This is a tracking ticket for all the tech debt we identified related to the build and test of the extension.

• remove direnv
• install mise
• install node/yarn with mise
• install python with mise
• install task with mise
• ensure e2e and ui tests do not download content on each run (enable caching on GHA)
• remove nyc with c8 as coverage tool (c8 is already used by extest and we have no reason to use multiple tools)
• remove console spam from extest output chore: filter unsupported NODE_* variables when calling VS Code redhat-developer/vscode-extension-tester#1859
• make extest downloads more resilient feat: make getFile retry downloads redhat-developer/vscode-extension-tester#1854
• remove sleep() from UI tests
• replace lcov.info with cobertura-coverage.xml format for coverage because we spotted that some tools fail to process lcov.info (lcov and gcovr cli tool). Those xml formats seems to be more standardized, even if they are bit smaller
• ensure that we have a coverage check limit configured for each type of testing, so we do not accidentally reduce coverage when making other changes. "check": true.
• Add junit reporting for all 4 types of tests
• Ensure coverage is correctly recognized by codecov.io and we do not accidentally break it

Investigate

• Possible replacement of yarn with npm + lerna (like extest does)
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