Refactor: Upgrade to Node 18 and clean up dependencies

[ehrhart]

This pull request includes upgrades and dependency changes:

• Set Node.js version requirement to >=18.0.0.
• Upgrade eslint-plugin-import to 2.29.1.
• Remove unnecessary dependency rimraf.
• Replace certain dependencies with native Node.js functions:
  • Use fetch instead of axios for making HTTP requests.
  • Use fs instead of is-valid-path for checking if a path is valid.
  • Use node:assert and node:test instead of ava and nock for testing.

This also results in a significant decrease in bundle size:

Package	Size	Minified	Gzipped
[email protected]	214.76 KB	59.86 KB	21.07 KB
sparql-transformer@refactor/dependencies-and-upgrade	27.22 KB (-87.33%)	9.87 KB (-83.51%)	4.07 KB (-80.69%)