luci-app-filemanager: Use read_direct to retrieve the file content

[rdmitry0911]

• This PR is not from my main or master branch 💩, but a separate branch ✅
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• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: x86/64, openwrt-23.05, Chrome, Firefox ✅
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Description: Use read_direct() to read the file that allows downloading and editing binary files

[systemcrash]

A couple of points for next version: If it's a big 'hex only' file, perhaps disable the 'Toggle ASCII' button. There's no point in generating an error there when we click it if we won't open in ASCII. Maybe that can even go into this fix.

[rdmitry0911]

disable the 'Toggle ASCII' button

I made some mess in this PR and finally closed it and opened a new one Your logic is correct and I put it there