@uppy/aws-s3: clarify and warn when incorrect buckets settings are used

[mifi]

fixes #5388

also:

• add new dev uploader s3-non-multipart, that will never use multipart, and s3 will auto-detect (which is the default anyways)
• add coment and slightly improve types

[github-actions]

Diff output files

diff --git a/packages/@uppy/aws-s3/lib/index.js b/packages/@uppy/aws-s3/lib/index.js
index d49a27a..b6c9105 100644
--- a/packages/@uppy/aws-s3/lib/index.js
+++ b/packages/@uppy/aws-s3/lib/index.js
@@ -500,12 +500,12 @@ export default class AwsS3Multipart extends BasePlugin {
           etag,
           location,
         } = headersMap;
-        if (method.toUpperCase() === "POST" && location === null) {
+        if (method.toUpperCase() === "POST" && location == null) {
           console.warn(
             "AwsS3/Multipart: Could not read the Location header. This likely means CORS is not configured correctly on the S3 Bucket. See https://uppy.io/docs/aws-s3-multipart#S3-Bucket-Configuration for instructions.",
           );
         }
-        if (etag === null) {
+        if (etag == null) {
           reject(
             new Error(
               "AwsS3/Multipart: Could not read the ETag header. This likely means CORS is not configured correctly on the S3 Bucket. See https://uppy.io/docs/aws-s3-multipart#S3-Bucket-Configuration for instructions.",