Modernize lsp-mode integration

* switch the company-backend used by lsp-mode from company-lsp to
company-capf (said to provide better performance and allows us to remove
the dependency on company-lsp)

* when available (on recent builds of Emacs 27+), lsp-mode buffers will
now use a larger than default value of `read-process-output-max`.
According to the lsp-mode performance guide, this can also significantly
improve lsp-mode performance

* flycheck support recently moved into lsp proper; with this commit,
the lsp module follows suit and prefers lsp-flycheck over
lsp-ui-flycheck

* if the new module flag `+no-peek` is set, doom-emacs will now use the xref
backend for reference and definition lookup instead of the lsp-ui-peek-*
backend offered by lsp-ui-mode

docs/modules.org

@@ -162,7 +162,7 @@ Small modules that give Emacs access to external tools & services.
 + gist - TODO
 + [[file:../modules/tools/lookup/README.org][lookup]] =+dictionary +docsets= - Universal jump-to & documentation lookup
   backend
-+ [[file:../modules/tools/lsp/README.org][lsp]] - TODO
++ [[file:../modules/tools/lsp/README.org][lsp]] =+peek= - TODO
 + macos - TODO
 + magit - TODO
 + make - TODO