Some minor bpaf fixes

[pacak]

• elp::args - use flag instead of switch + negate combo
• add /target to .gitignore

Describe your changes

minor improvements to bpaf code, it should be possible to clean it up more with 0.9 branch - show help by default, etc.

---

--experimental flag seems a bit strange. Name and help suggest that it will enable experimental diagnostics, while value it produces and how it is later used suggests that experimental diagnostics will be disabled:

crates/elp/src/bin/elp_parse_cli.rs:    cfg.disable_experimental = args.experimental_diags;
crates/elp/src/bin/lint_cli.rs:            cfg.disable_experimental = args.experimental_diags;

[facebook-github-bot]

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[facebook-github-bot]

@alanz merged this pull request in 2b07563.