feat: add getContents() for reading external markdown files into table descriptions

[AmmanuelT]

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.

[kolina]

/gcbrun

[kolina]

Thank you very much for your contribution!

Overall looks good, I've left some not very big comments to address.

Currently tests are failing due to expired BQ credentials, can you please rebase your PR onto #2139 so I can re-run them?

[AmmanuelT]

Thanks for the comments! Working on the fixes!

[AmmanuelT]

@kolina ready for another check 🫡

[kolina]

I'd add separator to this.rootDir here (if it already doesn't end with it) to make sure that file a/somepath.txt won't be considered inside a/somepath/ folder

[AmmanuelT]

I hadn't even considered that, good call! It's been added.