Reasonably big JSON files within JSONL can't be uploaded via web uploader

[xSAVIKx]

Describe the bug
Uploading JSONL file with somewhat big JSON files in it fails during the headers check with a weird Unterminated string in JSON at position error message.

The reason for that is using only first 1000 bytes from the file to check the headers and with somewhat bigger JSON files we're just somewhere in the middle of the first JSON still. I believe this is the related place:

timesketch/timesketch/frontend-ng/src/components/UploadForm.vue

Line 552 in 12f441f

reader.readAsText(file.slice(0, 10000))

I've added a redacted copy from a real file I wanted to upload. It's smth from AWS WAF logs, but I guess any other similar system may have comparable size of objects nowadays.

To Reproduce
Steps to reproduce the behavior:

1. Go to timelines upload form.
2. Select the attached file.
3. See error

Expected behavior
The JSONL file is processed and ingested.
Screenshots

Desktop (please complete the following information):

• OS: Ubuntu
• Browser: Chrome
• Version: 131.0.6778.108

Additional information
Here's the example file: timesketch-long-json.jsonl.txt
(I had to change the extension to .txt as GitHub does not allow uploading jsonl).
It has only 2 JSON objects in it. If you drop the second one it actually would be able to succeed.

Timesketch version: 20241129