fix: add FLAG num support for agglutinative languages

[tolgakaratas]

Summary

• The spell checker's affix parser used rune (single character) as the map key for affix rules, which broke dictionaries using FLAG num format
• Numeric flags like 14308,10482,4720 were parsed by reading only the first digit, making most affix rules unreachable
• This affected all agglutinative languages (Turkish, Hungarian, Finnish, etc.) whose Hunspell dictionaries use FLAG num with thousands of suffix groups

Changes

• Change AffixMap key from rune to string to support multi-character flags
• Change compoundMap key from rune to string
• Add parseFlags() method that correctly handles all Hunspell flag formats: ASCII, num, long, and UTF-8
• Update expand() to use parsed flag slices instead of rune iteration
• Update compound rule parsing in gospell.go

Impact

For the Turkish dictionary (tr_TR), this fix enables correct recognition of ~59,000 suffix groups and ~15.8 million inflected word forms that were previously unreachable. The tr_TR.aff file uses FLAG num with comma-separated numeric IDs.

Before: hunspell -d tr_TR -l recognizes "belediyeye", "adaletli", "ancak" — but Vale flags them as unknown.
After: Vale correctly recognizes these words using the same dictionary files.

Test plan

• New unit tests for parseFlags() covering ASCII, num, long, and UTF-8 formats
• New unit test for FLAG num affix parsing and expansion
• New integration test: newGoSpellReader with FLAG num dictionary
• Backward compatibility test: ASCII flag dictionaries still work correctly
• All 8 new tests pass

=== RUN   TestParseFlagsASCII
--- PASS: TestParseFlagsASCII (0.00s)
=== RUN   TestParseFlagsNum
--- PASS: TestParseFlagsNum (0.00s)
=== RUN   TestParseFlagsLong
--- PASS: TestParseFlagsLong (0.00s)
=== RUN   TestParseFlagsUTF8
--- PASS: TestParseFlagsUTF8 (0.00s)
=== RUN   TestFlagNumAffixParsing
--- PASS: TestFlagNumAffixParsing (0.00s)
=== RUN   TestFlagNumExpand
--- PASS: TestFlagNumExpand (0.00s)
=== RUN   TestFlagNumGoSpellReader
--- PASS: TestFlagNumGoSpellReader (0.00s)
=== RUN   TestASCIFlagBackwardCompatibility
--- PASS: TestASCIFlagBackwardCompatibility (0.00s)
PASS