Fix starting range in REPLCompletions (#49547)

The new completion for `var"` fields (#49294) failed
when the `var"` was at the end of the completion query,
e.g. in `WeirdNames().var"`. This is because we have the
following behavior:

```
julia> findprev(==('x'), "x", 1)
1

julia> findprev(==('x'), "x", 2)
```

REPLCompletions attempt to find `.` starting after the `var"`,
which in this case is at the end of the string. Of course,
the index was probably off by one anyway, because
we didn't want to match `var".`, but nevertheless,
I find this behavior surprising (ref also [1]).
For now, fix this by starting the search before the `var"`,
but we may want to revisit the `findprev` behavior also.

[1] https://github.com/JuliaLang/julia/pull/35742/files#r420945975

stdlib/REPL/src/REPLCompletions.jl

@@ -1021,7 +1021,7 @@ function completions(string::String, pos::Int, context_module::Module=Main, shif
  ok, ret = bslash_completions(string, pos)
  ok && return ret
  startpos = first(varrange) + 4
- dotpos = something(findprev(isequal('.'), string, startpos), 0)
+ dotpos = something(findprev(isequal('.'), string, first(varrange)-1), 0)
  return complete_identifiers!(Completion[], ffunc, context_module, string,
  string[startpos:pos], pos, dotpos, startpos)
  # otherwise...

stdlib/REPL/test/replcompletions.jl

@@ -1819,7 +1819,7 @@ let s = "var\"complicated "
  @test c == Any["var\"complicated symbol with spaces\""]
 end
 
-let s = "WeirdNames().var\"oh "
+for s in ("WeirdNames().var\"oh ", "WeirdNames().var\"")
  c, r = test_complete_foo(s)
  @test c == Any["var\"oh no!\"", "var\"oh yes!\""]
 end