modules/lsp: select relevant fields in keymaps table

We only use `mode`, `key`, `action`, and `options`; so explicitly
inherit those.

This means we don't serialise attrs that aren't needed at runtime,
such as `lspBufAction`.

tests/test-sources/modules/lsp.nix

@@ -86,13 +86,11 @@
                 {
                   action = vim.lsp.buf["definition"],
                   key = "gd",
-                  lspBufAction = "definition",
                   mode = ""
                 },
                 {
                   action = vim.lsp.buf["hover"],
                   key = "K",
-                  lspBufAction = "hover",
                   mode = ""
                 },
                 {