refactor!: rework results to remove redundant `flags` property and store value true for boolean options

[shadowspawn]

Leaner, meaner, cleaner!

See #70 for longer description (deleting flags in https://github.com/pkgjs/parseargs/issues/70#issuecomment-1070527258). See #80 for previous proposal to rename flags rather than delete, and this PR built on that.

The big change in this PR is removing the flags property in the results. flags is not encoding any additional information, and is not adding much. The lack of a compelling use case has made renaming it difficult!

In parse results:

• remove flags
• store true in values for boolean options (rather than undefined)

In README, a number of minor fixes after reading through examples carefully:

• update result property descriptions
• update -- description to match current implementation
• fix one example that got broken in a previous refactor
• replace stale "withValue" with "type:string"
• replace "flag" word with "option" or " boolean option" as appropriate
• other small tweaks to wording

[shadowspawn]

Got a couple of 👍 (thanks) but still looking for an approving review before can merge.

[shadowspawn]

I'm okay with this simplification, if we've thought things through and don't think it will limit any use-cases.

I worked through some core use cases in #38 using various patterns for the results, and didn't find any holes. (At the time the approach used in this PR was just included for interest!)

[bcoe]

@shadowspawn I think this is ready to go IMO, except it appears to have one failing test.

[shadowspawn]

I'll need to add type:* in various tests after the merge.

[bcoe]

I'll need to add type:* in various tests after the merge.

:+1: if you're feeling good about the PR, I can do my best to merge early this week and get our current state synced with the Node.js PR.

[shadowspawn]

Ready, barring more changes landing on main first. 😄