REPL: JuliaSyntax completions overhaul

This commit replaces the heuristic parsing done by REPLCompletions.completions
with a new approach that parses the entire input buffer once with JuliaSyntax.
In addition to fixing bugs, the more precise parsing should allow for new
features in the future.

Some features now work in more situations "for free", like dictionary key
completion (the expression evaluated to find the keys is now more precise) and
method suggestions (arguments beyond the cursor can be used to narrow the list).

The tests have been updated to reflect slightly differing behaviour for string
and Cmd-string completion: the new code returns a character range encompassing
the entire string when completing paths (not observable by the user), and the
behaviour of '~'-expansion has be tweaked to be consistent across all places
where paths can be completed.  Some escaping issues have also been fixed.

Fixes: #55420, #55518, #55520, #55842, #56389, #57611

Author: xal-0

stdlib/REPL/src/REPL.jl

@@ -86,6 +86,7 @@ import .LineEdit:
     PromptState,
     mode_idx
 
+include("SyntaxUtil.jl")
 include("REPLCompletions.jl")
 using .REPLCompletions