Provide API to tune the balancing factors used in conversion

[kanru]

Is your feature request related to a problem? Please describe.

酷音的「注音→詞」轉換引擎有一組規則用來計算最佳的斷詞方式

libchewing/src/conversion/chewing.rs

Lines 470 to 475 in 073e7c0

	let mut score = 0;
	score += 1000 * self.rule_largest_sum();
	score += 1000 * self.rule_largest_avgwordlen();
	score += 100 * self.rule_smallest_lenvariance();
	score += self.rule_largest_freqsum();
	score

這些權重應該要是可以調整的。

Describe the solution you'd like

提供 API 讓進階使用者可以預覽並調整權重

Describe alternatives you've considered

Use environment variable or config files.