feat(completion): complete flag values and stop offering file names

Every value completed to file names, because no command registered a
completion. Values without a completer now complete nothing, and only
path flags (--*-file, --input, --attachment, --asset, --file, --ssh-key,
--dir, --cwd) and arguments named file or directory complete paths.

--json completes the fields of the command, also after a comma. --host,
--user, --remote and -R complete from hosts.yml, Git remotes and the
instance. enumFlag and enumValues give a flag one list of values that
validation (case-insensitive) and completion share, and instance
completers for issues, pull requests, labels, milestones, users,
branches, tags, releases, organizations, teams, workflows, runs, secrets
and variables are ready for commands to attach. They give up after two
seconds, never prompt, retry or refresh an OAuth token, and drop values
with control characters.

tea completion bash now says to use eval "$(tea completion bash)",
because bash 3.2 on macOS cannot source <(...), and names the
bash-completion package; the zsh help says to run compinit first. The
README matches.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/app_test.go

@@ -208,6 +208,7 @@ func TestRootHelpGroupsCommandsAndTopics(t *testing.T) {
 		"\n  repo:          ",
 		"\nACTIONS COMMANDS\n  run:           ",
 		"\nADDITIONAL COMMANDS\n  admin:         ",
+		"\n  completion:    Generate shell completion scripts\n",
 		"\nALIAS COMMANDS\n  co:            Alias for \"pr checkout\"\n",
 		"\nHELP TOPICS\n  environment:   ",
 		"\nFLAGS\n  --help      Show help for command\n  --version   Show tea version\n",
@@ -302,6 +303,10 @@ func TestHelpTopicsAndUnknownTopics(t *testing.T) {
 		require.EqualError(t, err, want, args)
 		require.Empty(t, out.String(), args)
 	}
+	values, _ := complete(t, deps, out, "help", "e")
+	require.Equal(t, []string{"environment\tEnvironment variables and global flags", "exit-codes\tExit codes used by tea"}, values)
+	values, _ = complete(t, deps, out, "help", "issue", "l")
+	require.Equal(t, []string{"list\tList repository issues"}, values)
 }
 
 func TestVerbAndFlagAliases(t *testing.T) {
@@ -423,10 +428,6 @@ const (
 	branch rename
 	branch unprotect
 	branch view
-	completion bash
-	completion fish
-	completion powershell
-	completion zsh
 	config get
 	config list
 	config path