Add APIs and tests for currency sign options to CurrencyFormatter

[younies]

Summary

• Introduced CurrencySign enum to control formatting of negative currency values, supporting both standard and accounting styles.
• Updated CurrencyFormatterOptions to include a currency_sign field, defaulting to Standard.
• Added tests for accounting style formatting in test_en_us_accounting to implement the new behaviour in the next PR

Changelog

icu_experimental/currency: Add options for accounting currency sign

• New enum: CurrencySign
• New field CurrencyFormatterOptions::currency_sign
• New impl: impl From<CurrencySign> for CurrencyFormatterOptions

[sffc]

Nit: All options should be optional

Suggested change

	pub currency_sign: CurrencySign,
	pub currency_sign: Option<CurrencySign>,

Probably width should be optional, too.

[sffc]

Note: the changelog needs to follow a specific format. You had put:

• Introduced CurrencySign enum to support standard and accounting currency sign styles.
• Added currency_sign to CurrencyFormatterOptions, with Standard as the default.
• Added en-US accounting-style formatting tests.

I changed it, according to https://github.com/unicode-org/icu4x/blob/main/documents/process/changelog.md

Also, now that we have the ## Changelog section with the specific APIs, your ## Summary section (which doesn't need a header) should describe what the PR accomplishes at a high level.