Cannot import notebook if any cell name contains invalid characters

[mootari]

Describe the bug

Follow-up to #423

Notebooks that still contain invalid names cannot be imported because their compiled module code contains syntax errors. Examples:

• https://api.observablehq.com/@observablehq/sample-datasets.js?v=3
• https://api.observablehq.com/@observablehq/database-client-specification.js?v=3

[CobusT]

I think these will just have to be updated with valid names. Thanks for pointing them out.

[mootari]

For posterity: For the purpose of compilation invalid names are now treated as unnamed (which makes sense since they cannot be referenced in JS anyway). Anchor links continue to work.