Refactor CLI flags into modal options and add presets

This is a big commit that fixes a variety of issues and prevents fonts
from growing far too large without any benefits. Previously, converted
fonts would use multiple subtables for the `kern` table, but sadly those
are usually not read by renderers, so we now don't save al kern pairs,
but we prioritize.

The full list of changes can be found below.

---

Replace individual boolean flags with modal --kern and --hint options:

- --hint {skip,additive,overwrite,strip}: controls hinting behavior,
  including new ttfautohint support (additive/overwrite modes)
- --kern {add-legacy-kern,legacy-kern-only,skip}: replaces the old
  --skip-kobo-kern and --remove-gpos flags
- --preset {nv,kf}: bundled configurations for common workflows

Add upfront dependency checking (`ttfautohint`, `font-line`) so missing
tools are caught before any processing begins.

Fix GPOS Extension lookup (type 9) support: kern pairs were silently
missed in fonts that wrap PairPos subtables in Extension lookups.

Rework legacy kern table writing to respect format 0 size constraints.
The subtable length field is uint16, limiting a single subtable to
10,920 pairs. Since most renderers (including Kobo) only read the first
subtable, we write exactly one and prioritize pairs by Unicode range.

=> Basic Latin > Latin-1 Supplement > Latin Extended > rest

This way, the most commonly encountered kerning pairs are preserved
when truncation is needed (and it usually is, for quality fonts).

helpers/ttfconv.py

@@ -1,49 +0,0 @@
-#!/usr/bin/env python3
-
-# ttfconv.py
-
-# This script converts OTF fonts to TTF fonts using the font-tools library.
-# It processes a list of font files provided as command-line arguments.
-
-# The font-tools library must be installed: `pip install fonttools`.
-
-from fontTools.ttLib import TTFont
-import os
-import sys
-
-def convert_font(input_file_path):
-    """
-    Converts a font file to a TTF font file.
-    This function currently assumes the input is OTF and the output is TTF.
-
-    Args:
-        input_file_path (str): The path to the input font file.
-    """
-    if not os.path.exists(input_file_path):
-        print(f"❌ Error: The file '{input_file_path}' was not found.")
-        return
-
-    try:
-        output_file_path = os.path.splitext(input_file_path)[0] + ".ttf"
-        font = TTFont(input_file_path)
-        font.save(output_file_path)
-
-        print(f"✅ Converted: {os.path.basename(input_file_path)} -> {os.path.basename(output_file_path)}")
-
-    except Exception as e:
-        print(f"❌ An error occurred during conversion of '{os.path.basename(input_file_path)}': {e}")
-
-if __name__ == "__main__":
-    if len(sys.argv) < 2:
-        print("Usage: python font_converter.py <font_file1> <font_file2> ...")
-        print("Example: python font_converter.py MyFont.otf AnotherFont.otf")
-        print("You can also use a wildcard: python font_converter.py *.otf")
-    else:
-        for file_path in sys.argv[1:]:
-            if file_path.lower().endswith(".otf"):
-                convert_font(file_path)
-            else:
-                print(f"⚠️ Skipping '{file_path}': This script only converts OTF to TTF.")
-                print(f"To convert other formats, please provide the correct extension.")
-
-    print("\nProcessing complete.")