Make partial success of FlagSet.Parse possible (#155)

* Parse: mark as parsed, even on error

* UnknownFlagError

v4.0.0-alpha.4 (#122)

* Default value updates

* FlagSet struct tests

* Parse test failing (#118)

* Parse test passing (#118)

* WithFilesystem takes iofs.FS (#119)

* doc.go: fixup

* README: tweaks

* Tweaks

* Tweak

* Tweaks and more tests

v4-alpha.3 (#117)

* bool flag changes

* Major retooling

* Add Rewrap

* Rework testing and constructors

* Improve run-action-tests

v4-alpha.2 (#114)

* ffhelp: refactor default sections

* Huge WIP

* README: fixes and tweaks

* FlagSetAny

* Iteration

* Fix for CI

* Major continued refactoring

* Minor updates

* Improvements

* WIP

* WIP

* WIP

* ffenv: fix typo

* ffjson, fftoml: fix the same typo

* Comment cleanup

v4.0.0-alpha.1 (#113)

* Initial commit of ff v4 (rebase 1)

* Experiment: rename FlagSet to Flags

* examples/objectctl: add a test case

* Comment updates

* Fix comment

* Comment updates

* examples/textctl: minor changes to var names

* Minor renames to ParseContext fields

* Tweaks

* README updates

* ffhelp: overhaul

* Tweaks and renames

* Minor tweak

* ExampleParse_help

* README updates

* README updates

* Comment updates

* ffbasic + support

* internal: ffdata

* Cleanup

* Comment fixes

JSON, TOML, YAML: support for nested config data (#112)

* internal: add TraverseMap

* gofumpt

* JSONParseConfig

* internal: TraverseMap: map[any]any

* fftoml: don't break the API

* internal: TraverseMap: test for map[any]any

* fftest, ffyaml: nested node tests

* Can't remove ParseError types

* Can't remove StringConversionError type

WithFilesystem (#110)

* WithFilesystem

* configFileLookup was unused

* options.go was unused

* json[_test].go -> json_parser[_test].go

ffcli.DefaultUsageFunc: Support flag help placeholders (#106)

(This is an arguably uncontroversial part of the proposal in #105.
Its's a new feature, not a significant change in behavior.)

Currently, ffcli.DefaultUsageFunc prints "..." for any flag
that does not have a default value specified.
This produces less-than-effective help from DefaultUsageFunc.

This change retains the behavior of printing the default value as-is,
but if a default value is not provided,
it allows users to provide placeholder text
by wrapping a word inside the help text for a flag in backticks.

For example, given the following:

    fset.String("c", "" /* default */, "path to `config` file")

We'll get:

    -c config  path to config file

This matches the behavior of FlagSet.PrintDefaults,
and indeed it relies on the same flag.UnquoteUsage machinery for this.

This also has the nice side-effect of making a reasonable guess
at an alternative placeholder text instead of "...". For example:

    fset.Int("n", "" /* default */, "number of items")

    // Before: -n ...  number of items
    // Now:    -n int  number of items

Note that as implemented right now, the user supplied placeholder will
be used only if a non-zero default value was not supplied.
This was an attempt to retain as much of the existing behavior.
The proposal in #105, if you're open to it,
would change more of the output.

Attempt fixes for go 1.19 (#98)

* Attempt fixes

* fftoml: fix?

* Fixes

Add a naïve parser for .env files (#89)

* Add a naïve parser for .env files

* Remove dead code

* Cleanup