In this new structure, the main configuration for a LSP server is
provided by the plugin nvim-lspconfig.
Some of the config may be overridden by the configuration under
directory `after/lsp/xxx.lua`, where `xxx` is the lsp server name used
by nvim-lspconfig.
Note that it is necessary to put the custom lsp server configuration
under `after/` directory, in order to correctly override the config
provided by nvim-lspconfig.
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