patch 9.1.1042: filetype: just files are not recognized

Problem:  filetype: just files are not recognized
Solution: adjust filetype detection pattern, detect just shebang line,
          include just ftplugin, indent and syntax plugin
          (Peter Benjamin)

closes: #16466

Signed-off-by: Peter Benjamin <petermbenjamin@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/indent/just.vim

@@ -0,0 +1,51 @@
+" Vim indent file
+" Language:	Justfile
+" Maintainer:	Peter Benjamin <@pbnj>
+" Last Change:	2025 Jan 19
+" Credits:	The original author, Noah Bogart <https://github.com/NoahTheDuke/vim-just/>
+
+" Only load this indent file when no other was loaded yet.
+if exists("b:did_indent")
+  finish
+endif
+let b:did_indent = 1
+
+setlocal indentexpr=GetJustfileIndent()
+setlocal indentkeys=0},0),!^F,o,O,0=''',0=\"\"\"
+
+let b:undo_indent = "setlocal indentexpr< indentkeys<"
+
+if exists("*GetJustfileIndent")
+  finish
+endif
+
+function GetJustfileIndent()
+  if v:lnum < 2
+    return 0
+  endif
+
+  let prev_line = getline(v:lnum - 1)
+  let last_indent = indent(v:lnum - 1)
+
+  if getline(v:lnum) =~ "\\v^\\s+%([})]|'''$|\"\"\"$)"
+    return last_indent - shiftwidth()
+  elseif prev_line =~ '\V#'
+    return last_indent
+  elseif prev_line =~ "\\v%([:{(]|^.*\\S.*%([^']'''|[^\"]\"\"\"))\\s*$"
+    return last_indent + shiftwidth()
+  elseif prev_line =~ '\\$'
+    if v:lnum == 2 || getline(v:lnum - 2) !~ '\\$'
+      if prev_line =~ '\v:\=@!'
+        return last_indent + shiftwidth() + shiftwidth()
+      else
+        return last_indent + shiftwidth()
+      endif
+    endif
+  elseif v:lnum > 2 && getline(v:lnum - 2) =~ '\\$'
+    return last_indent - shiftwidth()
+  elseif prev_line =~ '\v:\s*%(\h|\()' && prev_line !~ '\V:='
+    return last_indent + shiftwidth()
+  endif
+
+  return last_indent
+endfunction