This removes semicolons at the end of the line and collapses
consecutive ones, while replacing meaningful semicolons with newlines.
I.e.
```fish
echo;
```
becomes
```fish
echo
```
but
```fish
echo; echo
```
becomes
```fish
echo
echo
```
Fixes#5859.
Fish keywords can be quoted and split across lines. Prior to this change
`fish_indent` would retain such odd, obfuscated, formatting. This change
results in all keywords being converted to their canonical form.
This required fixing a bug: the keyword member of parse_node_t wasn't being
populated. This hadn't been noticed prior to now because it wasn't used.
Fixes#2921
Modify `fish_indent` to emit redirections without a space before the target of
the redirection; e.g., "2>&1" rather than "2>& 1" as the former is clearer to
humans.
Fixes#2899
