feat: use existing servers & define default servers

[GR3YH4TT3R93]

this commit enables the use of pre-installed LSP servers. It requires that the auto-install logic is broken out into ensure_installed in order to keep auto install functionality while allowing for servers to be set up in a unified manner.

• change vim.tbl_deep_extend order to ensure opts overrides defaults
• use ensure_installed table to define ensure_installed for mason_lspconfig
• add set up for any servers not managed by mason
• minor formatting (add missing commas)
• include setup instructions in README