Closes #1723

Allows users to source table descriptions from external markdown files, keeping .sqlx files smaller and enabling reusable, renderable descriptions across the project.

Usage:

config {
  type: "table",
  description: getContents('./my_table_description.md')
}
SELECT ...

Changes:

• core/compilers.ts : .md files now compile to exports.contents = ...``
• core/session.ts : new getContents(filePath) method; resolves the path relative to the calling file using the call stack, then builds an absolute path for nativeRequire
• core/main.ts : getContents bound to the global scope
• cli/vm/compile.ts + testing/run_core.ts: "md" added to sourceExtensions so vm2 applies the compiler to .md files
• core/main_test.ts + core/compilers_test.ts : tests for happy path and missing file error

Notes:

• The method is currently named getContents, inspired by the suggestion in the issue thread, but happy to rename before merge if the maintainers have a preference!
• When getContents references a file that doesn't exist, the error surfaces as a VMError: Cannot find module '/absolute/path/to/file.md' compilation error. A cleaner message could be produced by intercepting in dataformCompile in main.ts also happy to add this if preferred, just wanted to flag the tradeoff before doing extra work.