selector syntax errors point to wrong char if at end of input

[yshavit]

Example 1: Unexpected end of input:

$ cat example.md | mdq '- ['

Syntax error in select specifier:
┃ - [
┃   ↑ unexpected end of input

The arrow should point to the position after the [:

┃ - [
┃    ↑ unexpected end of input

Example 2: Unexpected end of input when we expect a specific char:

$ cat example.md | mdq '- [x'

Syntax error in select specifier:
┃ - [x
┃    ↑ expected "]"

Again, the arrow should point at the missing char:

┃ - [x 
┃     ↑ expected "]"

Additional notes:

• The above examples are different error reason: one is UnexpectedEndOfInput, the other is Expected(']'). That means I can't just match my way out of this; I'll need to actually think about what's going on. :-)
• While I'm fixing this, I should see if there's a better approach to the err.position.column - 1 hack in main.rs where I print the syntax error.
• This was introduced/discovered in various parser improvements #64, but it's not important enough for me to block that PR on this bug.