feat: support new 0.19 language features

[hpopp]

Supports Koja 0.19.

Added

• Syntax highlighting, auto-indent, and folding for test "description" blocks, and highlighting for the assert statement.
• Syntax highlighting for function and constant aliases (alias JSON.decode). The package head is a namespace, not a type.
• A Testing view. test blocks under a project's src/ and test/ appear in the tree, a run executes koja test --reporter json for the project, and each result shows inline. A failed assert shows a diff of both sides at the assertion line.
• Koja: Test Project command, which runs koja test for the current project in the shared terminal.
• The extension activates when the workspace contains a koja.toml, so tests appear before a .koja file is opened.

Changed

• README lists the language-server features that koja-lsp 0.19 answers, which now include signature help, find references, rename, document highlight, inlay hints, workspace symbols, and folding.