Add project manager features

[CyberMango]

Add features for managing projects in vim:

• Open a project workspace, with project dir being cwd.
• Create new projects with a certain template and pre-made files, like .ccls files for cpp.
• Browse existing projects.

Might use:

• :mksession and vim -S .
• vim-session plugin.
• some project manager plugin.
• Some plugin I'll make?