refactor!: remove lspconfig dependency

[jose-elias-alvarez]

Closes #29.

This seems to be working for simple project structures but needs more testing. I'd at least want to wait until 0.8 is released to merge this, at which point vim.lsp.start should hopefully have had time to settle into the ecosystem (as far as I can tell from a Google search, no GitHub projects are using it yet).

The LSP ecosystem is so dependent on lspconfig, but this is perhaps 90% of the way there with a fraction of the code and no external dependencies. If nothing else, having more vim.lsp.start users can help uncover areas where we're lacking parity and hopefully drive contribution to core (for example, I plan on investigating if there's anything we can do about the lack of an lspconfig-independent client restart command).

Edit: this is a breaking change because users will no longer have access to lspconfig-specific features like user commands and :LspRestart.