nvim: plugin/shell: Add key mapping to run jq on visual selection

While at it, add some explanation to the functions for posterity.

nvim/.config/nvim/plugin/shell.vim

@@ -1,3 +1,5 @@
+" Helps with running a shell command and opens it result in another tab. Most
+" helpful with GStreamer logs to run filters like rg 'pipeline|udpsink' %.
 function! s:ExecuteInShell(command) " {{{
   let command = join(map(split(a:command), 'expand(v:val)'))
   let winnr = bufwinnr('^' . command . '$')
@@ -9,7 +11,25 @@ function! s:ExecuteInShell(command) " {{{
   silent! execute 'nnoremap <silent> <buffer> q :q<CR>'
   echo 'Shell command ' . command . ' executed.'
 endfunction " }}}
-
 command! -complete=shellcmd -nargs=+ Shell call s:ExecuteInShell(<q-args>)
 
+" Make a visual selection first. This function then opens a scratch buffer
+" in a new tab, pastes the visual selection and runs jq on the buffer
+" replacing its contents. Helps with analysing portions of pino logs processed
+" by pino-pretty.
+function! s:JqScratch()
+  tabnew
+  noswapfile hide enew
+  setlocal buftype=nofile
+  setlocal bufhidden=hide
+  setlocal nobuflisted
+  file scratch
+  norm p
+  silent! execute '%!jq'
+  silent! execute 'set ft=json'
+  silent! execute 'nnoremap <silent> <buffer> q :bw<CR>'
+endfunction
+command! JqScratch call s:JqScratch()
+
+nnoremap <Leader><Leader>j :JqScratch<CR>
 nnoremap <Leader><Leader>s :Shell rg '' %<Left><Left><Left>