Build plugin with qt6 and qtc6

[gikari]

This is work in progress.

TODO

• [ ] Make the block selection work

Notes

• It uses a fork of neovim-qt, that works with qt6. Maybe the fork should be incorporated into an GitHub organization?
• Qt Creator 6 works only with the plugin compiled using Qt 6.2 for some reason

Test plan

1. The project should compile with Qt 6.2 and Qt Creator 6.0.2
2. Selections (v, V and Ctrl-V ones) in the editor should work.

[gikari]

To avoid noise, I will temporarily close the PR. Once I figure out the CI, I will reopen it.