WithEnvVarNoPrefix() (Deprecated) -> WithEnvVars()

(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.

* WithFilesystem

* configFileLookup was unused

* options.go was unused

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

* ffcli: DefaultUsageFunc: add DESCRIPTION with ShortHelp

* ffcli: fix tests

* 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