add fish shell based completer (#4569)

* feat: add fish shell based completer

* refactor: use filter-func to filter results

news/fish-completer.rst

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+**Added:**
+
+* `xontrib.fish_completer` is available to complete using `fish` shell.
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

xontrib/fish_completer.py

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+from xonsh.completers import completer
+from xonsh.completers.tools import (
+    RichCompletion,
+    contextual_command_completer,
+    get_filter_function,
+)
+
+import subprocess as sp
+from xonsh.built_ins import XSH
+from xonsh.parsers.completion_context import CommandContext
+
+
+def create_rich_completion(line: str):
+    line = line.strip()
+    if "\t" in line:
+        cmd, desc = map(str.strip, line.split("\t", maxsplit=1))
+    else:
+        cmd, desc = line, ""
+    return RichCompletion(
+        str(cmd),
+        description=str(desc),
+        append_space=True,
+    )
+
+
+@contextual_command_completer
+def fish_proc_completer(ctx: CommandContext):
+    """Populate completions using fish shell and remove bash-completer"""
+    args = [arg.value for arg in ctx.args] + [ctx.prefix]
+    line = " ".join(args)
+    args = ["fish", "-c", f"complete -C '{line}'"]
+    env = XSH.env.detype()
+    output = sp.check_output(args, env=env).decode()
+    filter_func = get_filter_function()
+
+    if output:
+        for line in output.strip().splitlines(keepends=False):
+            comp = create_rich_completion(line)
+            if filter_func(comp, ctx.prefix):
+                yield comp
+
+
+completer.add_one_completer("fish", fish_proc_completer, "<bash")