introduce mxlint.io functions for javascript rules

[xiwenc]

fixes #89

The mxlint object provides utility functions for JavaScript rules:

• mxlint.io.readfile(path): Reads a file and returns its contents as a string.
  The path is resolved relative to the workingDirectory.
• mxlint.io.listdir(path): Lists the contents of a directory and returns an array of filenames.
  The path is resolved relative to the workingDirectory.
• mxlint.io.isdir(path): Returns true if the path is a directory, false otherwise.
  The path is resolved relative to the workingDirectory.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/mxlint/mxlint-cli/lint	67.03% (+2.58%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/mxlint/mxlint-cli/lint/lint_javascript.go	83.33% (+10.61%)	126 (+60)	105 (+57)	21 (+3)	🎉
github.com/mxlint/mxlint-cli/lint/rules.go	62.66% (+0.24%)	158 (+1)	99 (+1)	59	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/mxlint/mxlint-cli/lint/lint_javascript_test.go