fix: more CJS compatibility

[JasonWeinzierl]

• Change package.json type to CJS for more compatibility.
• Replace unbuild with tsup for more default export ergonomics.
  • unbuild was fine as long as we added a patch script to export default in CJS. However that's a bit unexpected in CJS; you would expect to just do const rxjsX = require('eslint-plugin-rxjs-x'); without .default tacked onto the end. tsup lets us build different output for CJS that's more ergonomic for CJS users.
  • Had to remove the assert { type: 'json' } since esbuild doesn't support it combined with named exports.
• Only support decamelize v5.
  • v6 is ESM-only, which would be fine if users use this plugin in an ESM eslint config. However if they're using a CJS eslint config and they install this package, their package manager will likely pull in decamelize v6 and cause a error trying to import it. Downgrading helps avoid that confusion; advanced users who need v6 can use something like yarn resolutions to override.

[github-actions]

LCOV of commit 793a0df during .github/workflows/ci.yml #19

Summary coverage rate:
  lines......: 96.2% (3249 of 3376 lines)
  functions..: 92.9% (196 of 211 functions)
  branches...: 91.2% (613 of 672 branches)

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/index.ts                           | 100%     97|    -     0|    -      0