feat: add BSL (1C:Enterprise) language server support

[Romandredan]

Summary

Adds support for BSL (1C:Enterprise / OneScript) language via integration with
bsl-language-server v0.29.0.

Changes

• New BSLLanguageServer class in src/solidlsp/language_servers/bsl_language_server.py
• Registered Language.BSL enum with *.bsl, *.os filename matching
• Test fixture repo and integration tests under test/solidlsp/bsl/
• Updated README and CHANGELOG

Requirements

• Java 11+ on PATH
• bsl-language-server JAR is downloaded automatically on first use (verified by SHA-256)

Capabilities supported

• Document symbols
• Go-to-definition
• Find references
• Hover
• Rename (basic)

Tested on

• Windows 11
• Smoke-tested on Standard Subsystems Library (BSP) — .bsl files processed without errors