Upgrade to v0.12.x, remove nvim-treesitter

Switch to tree-sitter-manager.nvim for managing tree-sitter parsers and
queries. Also fixes upgrade issues.

plugin/ui.lua

@@ -1,17 +1,3 @@
--- Customise LSP UI
-vim.lsp.handlers['textDocument/hover'] = vim.lsp.with(
-  vim.lsp.handlers.hover, {
-    border = 'rounded',
-    title = 'Hover',
-  }
-)
-vim.lsp.handlers['textDocument/signatureHelp'] = vim.lsp.with(
-  vim.lsp.handlers.signature_help, {
-    border = 'rounded',
-    title = 'Signature Help',
-  }
-)
-
 -- Customise diagnostic UI
 vim.diagnostic.config({
   float = {