Neovim v0.7 Update

[axieax]

• [ ] Use new functions (nvim_buf_get_text, nvim_add_user_command)
• [x] Add v0.7 to pipeline for testing
• [ ] Pre v0.7 compat (v0.6 tag)

[axieax]

nvim v0.7 commands unnecessary thus no breaking changes required for supporting nvim 0.6.