fix(shell): parse fish aliases and abbreviations

[ItzTas]

closes #153

FishAdapter.ScanAliases sent config.fish through ScanPosixAliases, which reads bash/zsh syntax, so on a normal fish config you get almost nothing back and what you do get can be wrong. abbr wasn't read at all

alias clear 'clear -x'
abbr -a gl git log
if type -q lsd
    alias ls 'lsd --icon=always'
else
    alias ls 'ls --color=auto'
end
alias g='git'

before:  map[string]string{"'lsd --icon":"always'"}
after:   aliases map[clear:clear -x  g:git  ls:lsd --icon=always]
         abbrs   map[gl:git log]

The four problems from the issue, plus one that only showed up against a real config:

• alias name value has no = to split on so those lines were dropped, and when the value carried its own = the line survived but split in the wrong place. Both forms parse now, with -s/--save and -- handled (1dd9d53)
• blocks were counted with if/else/fi. fish closes everything with end, and that includes for/while/switch/begin/function, so counting only if lets an inner end pop the enclosing one and the fallback arm starts overwriting the preferred one. It's a stack now, first arm still wins, switch/case included (1dd9d53)
• only config.fish was read. Now functions/*.fish, conf.d/*.fish and config.fish, in that precedence order, since an autoloaded function only loads while its name is still undefined. $__fish_config_dir is resolved once per process, same trick as ZDOTDIR (1dd9d53)
• abbr parses in the forms people write: -a/--add, implicit add, --, --position, --set-cursor, -g. --regex and --function are skipped since neither the name nor the expansion is in the text, and a name the shell computes gets dropped instead of recorded as a literal. They reach the menu with their own source and icon, which fits the split from Add semantic, configurable icons for suggestion types #60 (6cec6ff)
• the fifth one: following source needed set and for resolved too. My own config names its parts in a list and sources them from a loop, so without that the parser never reaches a single alias file (1dd9d53)

for file in $files
    set -l config_file "$__fish_config_dir/config/$file.fish"
    path is -f -- "$config_file"; and source "$config_file"
end

The walker was split out first (d8fa4a1) so the posix path and the fish one share the source following and cache stamping instead of duplicating both

Note

aliases and abbrs come out of one walk and share one cache entry. Two scans with different entry points would evict each other and re-read the whole tree on every keystroke

abbrs aren't wired into core.expand-alias: fish expands them itself on space, and a --set-cursor abbr would get its marker typed literally

integration/shell/fish_test.go covers the alias and abbr forms, arm preference with nested blocks, statement separators, alias --save function files, the autoloaded dirs, loop-sourced files, and cache invalidation when a file lands in conf.d/. Against my real config it finds 27/27 aliases and 39/39 plain abbrs; the other 50 are the --command-scoped ones built by that loop with (string split ...) names, which no static parser can resolve. just test, just lint and go vet ./... are clean