ci: Add Biome as code formatter and linter

[sorairolake]

The code formatter and linter can be run locally as follows:

# Run linter
$ npx @biomejs/biome lint
# Run code formatter
$ npx @biomejs/biome format --write

This pull request combines both the code formatter and linter into a single tool, so files related to ESLint are removed. Also, since Travis CI¹ does not seem to be used, so remove files related to it.

Partially solves #290

Footnotes

1. .travis.yml defines flake8 and eslint. ↩

[sorairolake]

Moving to #293