Error recovery for unexpected continuation keywords

Consider
```
julia> JuliaSyntax.parse(JuliaSyntax.GreenNode, "if true; x ? true : elseif true end")[1]
1:35 │[toplevel]
1:35 │ [if]
1:2 │ if
3:3 │ Whitespace
4:7 │ true ✔
8:26 │ [block]
8:8 │ ;
9:26 │ [if]
9:9 │ Whitespace
10:10 │ Identifier ✔
11:11 │ Whitespace
12:12 │ ?
13:13 │ Whitespace
14:17 │ true ✔
18:18 │ Whitespace
19:19 │ :
20:20 │ Whitespace
21:26 │ [error] ✘
21:26 │ elseif ✔
27:27 │ Whitespace
28:31 │ [error] ✘
28:31 │ true ✔
32:32 │ Whitespace
33:35 │ end
```
This special case is fixed by #77 by punting the `elseif` into the containing block instead:
```
julia> JuliaSyntax.parse(JuliaSyntax.GreenNode, "if true; x ? true : elseif true end")[1]
1:35 │[toplevel]
1:35 │ [if]
1:2 │ if
3:3 │ Whitespace
4:7 │ true ✔
8:19 │ [block]
8:8 │ ;
9:19 │ [if]
9:9 │ Whitespace
10:10 │ Identifier ✔
11:11 │ Whitespace
12:12 │ ?
13:13 │ Whitespace
14:17 │ true ✔
18:18 │ Whitespace
19:19 │ :
20:19 │ error ✘
20:20 │ Whitespace
21:32 │ [elseif]
21:26 │ elseif
27:27 │ Whitespace
28:31 │ true ✔
32:32 │ [block]
32:32 │ Whitespace
33:35 │ end
```
but of course that naive solution only works if there is only one missing or extraneous token, so `"if true; x ? true : foo ))))) elseif true end"` will break it again.

Generally, this should be solvable by an arbitrarily long look-ahead for continuation keywords, but I really don't like that solution (and it might not even work in all cases).