feat: add support for iShares in Switzerland and update region handling

[yevklym]

This pull request adds support for the Swiss region (ch) to the iShares provider, improves normalization of Swiss number formats, and extends exchange normalization and documentation. The main changes are grouped below.

Swiss region (ch) support for iShares

• Added a new configuration for the Swiss region (ch) in ishares, including URLs, currency, exchange, and mappings for asset classes, sectors, and locations.
• Updated documentation and region listings to indicate support for ch (Switzerland) in both README.md and code comments. [1] [2]
• Expanded region test coverage to include ch and other supported regions in client_test.go.

Exchange normalization

• Added support for the SIX Swiss Exchange (SIX) in exchange enums and normalization logic, including recognition of "SIX" and "SIX SWISS EXCHANGE". [1] [2]
• Improved NYSE normalization to handle additional variants of the name.

Number format normalization

• Improved Swiss number format normalization by handling both straight and curly apostrophes as thousands separators.
• Added tests for curly apostrophe handling in Swiss number formats.

[Copilot]

Pull request overview

This PR adds iShares Switzerland (ch) support and extends normalization for Swiss-specific exchange and number formats within the ETF provider implementation.

Changes:

• Adds Swiss iShares region configuration and updates supported-region documentation.
• Adds ExchangeSIX plus iShares exchange aliases for SIX and additional NYSE variants.
• Extends Swiss number normalization to strip curly apostrophe thousands separators and expands related tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents ch as a supported iShares region.
enums.go	Adds the SIX Swiss Exchange enum value.
internal/providers/ishares/client.go	Updates package documentation with the full supported iShares region list.
internal/providers/ishares/client_test.go	Expands supported-region construction tests to include UK, FR, and CH casing.
internal/providers/ishares/config.go	Adds the Swiss iShares region configuration.
internal/providers/ishares/normalize.go	Adds normalization aliases for NYSE variants and SIX.
internal/providers/ishares/column_resolver.go	Strips additional curly apostrophe thousands separators.
internal/providers/ishares/column_resolver_test.go	Adds coverage for one curly apostrophe Swiss number format.