## Add keyword arguments for UI primitives and user-defined functions

### Summary

This PR introduces Ruby/Python-style keyword arguments to Faust, enabling more readable and self-documenting code for both UI primitives and user-defined functions:

```faust
// UI primitives
hslider("volume", init: 0.5, min: 0.0, max: 1.0, step: 0.01)

// User-defined functions
mix(a, b, ratio) = a * (1-ratio) + b * ratio;
process = mix(dry, wet, ratio: 0.7);
```

### Motivation

Faust's UI primitives require multiple positional arguments that are easy to confuse:

```faust
// Which is init? min? max? step?
hslider("volume", 0.5, 0.0, 1.0, 0.01)
```

Similarly, user-defined functions with multiple parameters benefit from named arguments for clarity at call sites.

### Syntax

#### UI Primitives

```faust
// Before: positional only
hslider("volume", 0.5, 0.0, 1.0, 0.01)

// After: keyword arguments (any order)
hslider("volume", init: 0.5, min: 0.0, max: 1.0, step: 0.01)
hslider("volume", min: 0.0, max: 1.0, init: 0.5, step: 0.01)
```

Supported: `hslider`, `vslider`, `nentry`, `hbargraph`, `vbargraph`

#### User-Defined Functions

```faust
filter(freq, q, gain) = ...;

// Mixed positional + kwargs
process = filter(1000, q: 0.7, gain: 1.0);

// All kwargs (any order)
process = filter(gain: 1.0, freq: 1000, q: 0.7);
```

**Constraints (Ruby-style):**
- Keyword arguments must come after positional arguments
- All required parameters must be provided
- Unknown keywords produce clear error messages

### Error Messages

```
error: hslider: unknown keyword 'foo'
Valid keywords are: init, min, max, step

error: hslider: missing required keyword 'step'
Required keywords are: init, min, max, step

error: unknown keyword argument 'c'
```

### Implementation

- **Lexer**: Added `KWARG` token (pattern: `{ID}":"`)
- **Parser**: Added `kwarg`, `kwarglist` grammar rules
- **Boxes**: Added `BOXKWARG` box type for keyword argument representation
- **Eval**: Modified `applyList()` to resolve kwargs to parameter positions

### Testing

Pass tests:
- `kwargs-ui-primitives.dsp` - all UI primitive variants
- `kwargs-functions.dsp` - user-defined function calls
- `kwargs-mixed.dsp` - combining both in one program

Error tests:
- `kwargs-unknown-ui.dsp` - unknown keyword for UI primitive
- `kwargs-missing-ui.dsp` - missing required keyword
- `kwargs-unknown-func.dsp` - unknown keyword for function

### Note on diff size

The large diff in `faustlexer.cpp`, `faustparser.cpp`, and `faustparser.hpp` is due to these being **generated files**. The actual source changes are:

| File | Lines changed |
|------|---------------|
| `faustlexer.l` | +1 |
| `faustparser.y` | +130 |
| `boxes.cpp` | +14 |
| `boxes.hh` | +7 |
| `eval.cpp` | +114 |
| `global.cpp` | +1 |
| `global.hh` | +1 |

### Backward Compatibility

Fully backward compatible - existing positional syntax continues to work unchanged.

### Future Work

This feature is designed to integrate with default parameters (separate PR):

```faust
filter(freq, q=0.707) = ...;
process = filter(freq: 1000);  // q defaults to 0.707
```