Emacs support

[jcs090218]

Improve documentation

Link

Mention Emacs support after https://pgtools.dev/#neovim.

Describe the problem

New LSP client for Emacs.

Describe the improvement

Mention Emacs client from lsp-mode.

Additional context

• The related documentation: https://emacs-lsp.github.io/lsp-mode/page/lsp-postgres/.
• The actual implementation: https://github.com/emacs-lsp/lsp-mode/blob/master/clients/lsp-postgres.el.

[jcs090218]

Completed in #378.