Refined parser error messages for flags given `=` values (#14)

This PR improves the clarity of argument parser messages when a boolean
flag is incorrectly provided with a value using `=`.

Previously, cases like `-f=val` or `--flag=value` would produce
misleading messages such as ‘unknown option '-f'’ even if the flag
exists. Now, the parser reports more precise messages:

- `unexpected '=' for option '-f'`
- `unexpected '=' for option '--flag'`
- `unexpected '=' in combined short options '-ff'`

Count number of occurrences for flags (#11)

`ParsedArgs` now stores the number of times boolean options (flags) were
encountered in the command line.

Example: `-vvvv` or `-v -v -v -v` will return `4` for `flag("-v")` as an
unsigned integer.

To check whether a flag is present (i.e. > 0 occurrences), use
`hasFlag("-v")`.

## Breaking Change

The old `bool flag(string | Flag)` method has been renamed to `bool
hasFlag(string | Flag)`.

The new method `uint flag(string | Flag)` is different and works as
described in this PR and in the documentation.

Fix bug introduced by feature in #10 (#12)

The feature checks for flags grouping like `-abc` but falsely interprets
sequences like `-a=value` as a group of flags.

Added support for grouping boolean options by short name (#10)

Flags (boolean options) like `-a -b -c` can be grouped like `-abc` (in
any order). Only works for flags/booleans and only for short options.

Example:

```d
new Program("cmd")
    .option("-f, --foo", "foo")
    .option("-b, --bar", "boo")
    .option("-B, --baz", "foo")
    // …
```

The flags can be set to true like so:

```sh
cmd -fbB
# equivalent to:
cmd -f -b -B
```

When reading the parsed flags, they are still read separately:

```
auto foo = args.flag("-f");
auto bar = args.flag("-b");
auto baz = args.flag("-B");
```

Fix flag explicit selection with `-` and `--` (#9)

Store flags with their full name (including `-` or `--`) to enable
explicit selection by short (`-`) and long (`--`) name, just like
options (named parameters).

Check `arg` is not empty before accessing `front()` (#8)

Fixes bug with 0-length args, e.g.

```
./cli ""
```

Improved README and examples (#6)

Initialise project (#1)

TODO:

- [x] Finish the `printHelp()` method
- [x] Add CI
- [x] Examples in README
- [x] Documentation (outside of source)

fix typo

added CI