statusline: reimplement default, include LSP progress

[justinmk]

Problem

the default 'statusline' is implemented in C, it doesn't use the 'statusline' DSL. And it isn't readable as a 'statusline' expression, so user configs/plugins can't augment it.

Expected behavior

• remove the C impl of 'statusline'. reimplement it as a 'statusline' expression
• show LspProgress in the default 'statusline' feat(lsp): redrawstatus on progress notifications #28569
• it should continue to be minimal and fast, but consider other useful things we can add

Related

• implement default titlestring using statusline #1248
• nvim_eval_statusline() can return the default statusline #28444