Add ALEJobStarted User autocommand event

The ALELintPre and ALELintPost autocommand events are currently being
used by lightline-ale to refresh the status line and check the linter
status for a current buffer. One of the plugin's checks looks to see if
linters are currently running, via ale#engine#IsCheckingBuffer(). This
currently only works partially in certain situations. In my particular
case, working with Go files, this only seems to function properly when a
file is initially opened. Saving a file does not correctly update the
status.

This seems to be due to the fact that ALELintPre actually runs before
any jobs are carried out, making it plausible that hooking into
ALELintPre for the purpose of checking to see if there are any currently
running linters for a buffer is unreliable as it would be prone to
pretty obvious race conditions.

This adds a new User autocommand, ALEJobStarted, that gets fired at the
start of every new job that is successfully run. This allows a better
point to hook into checking the linter status of a buffer using
ale#engine#IsCheckingBuffer() by ensuring that at least one job has
started by the time IsCheckingBuffer is run.

autoload/ale/engine.vim

@@ -586,6 +586,8 @@ function! s:RunJob(options) abort
         \   'output': [],
         \   'next_chain_index': l:next_chain_index,
         \}
+
+        silent doautocmd <nomodeline> User ALEJobStarted
     endif
 
     if g:ale_history_enabled

doc/ale.txt

@@ -2452,7 +2452,14 @@ ALEFixPost                                                 *ALEFixPost-autocmd*
         autocmd User ALELintPre  let s:ale_running = 1 | redrawstatus
         autocmd User ALELintPost let s:ale_running = 0 | redrawstatus
     augroup end
+
 <
+ALEJobStarted                                           *ALEJobStarted-autocmd*
+
+  This |User| autocommand is triggered immediately after a job is successfully
+  run. This provides better accuracy for checking linter status with
+  |ale#engine#IsCheckingBuffer()| over |ALELintPre|, which is actually
+  triggered before any linters are executed.
 
 ===============================================================================
 10. Special Thanks                                         *ale-special-thanks*