docs: add JSDoc to the public API

[UlisesGascon]

Adds JSDoc to the public surface: multer(options) with the current option set (dest/storage, fileFilter, limits incl. fieldNestingDepth and fieldArrayIndexLimit, preservePath, defParamCharset), .single/.array/.fields/.none/.any, diskStorage/memoryStorage, the storage-engine contract and MulterError (code, field, filename, list of codes). Internals are left as they are.

Supersedes #1284, which documented the 2.0 code and needed a rewrite after 2.3.0; its author is credited as co-author.

Closes #1284
Closes #1283

[coveralls]

coverage: 99.724%. remained the same — docs/jsdoc-public-api into main

[kilisamemarisaaa]

The File typedef does not match the object passed to the two public callbacks that reference it. File.size is currently required and stream is absent, but fileFilter runs before either property exists, and StorageEngine._handleFile receives file.stream while size is still absent.

I verified the lifecycle with a custom storage engine on the current implementation:

fileFilter:  stream=false, size=false
_handleFile: stream=true,  size=false
req.file:    stream=false, size=true

This also conflicts with the _handleFile description here ("Consumes file.stream") and with StorageEngine.md, while editor/type consumers would report the real storage-engine API as missing. Could the callback inputs use stage-specific typedefs (for example a base incoming file plus a storage file with stream), or otherwise make size optional and declare stream for the _handleFile parameter?