ESLint v9

[danbillson]

The key part of this change is the addition of this eslint rule:

'import/extensions': ['error', 'always', { js: 'always' }],

which enforces that all imports and exports contain the .js extension as this can easily be missed and can break certain builds as seen in #76

Other updates

• rename tsconfig.base.json to tsconfig.json - this was required for the eslint upgrade which couldn't find a root tsconfig