Improve UI/UX of the multipart tab

[NghiaTranUIT]

Description

• Add a Preview and Export button
• Preview allows the user to quickly preview the selected part with a separated view. Support JSON highlighting and prettify if need
• Column is resizable, toggle to on/off
• persist to the next launch

Why this feature/change is important?

• Easier to work with the large multiparts data

[NghiaTranUIT]

• Fixed: https://assets.proxyman.com/beta/Proxyman_6.12.0_Better_multipart_ui.dmg