Link to local lua_api.md instead of GitHub-hosted version

[mark-wiemer]

The local version will be much faster to navigate to and can be done in the same context, e.g. VS Code, instead of opening a separate browser. Obviously local version would work offline as well.

Note that the lua_api.md file is currently packaged with the VSIX extension but never actually referenced--an upcoming PR will remove that file to trim the extension size by a good amount :)

[GreenXenith]

Is the idea to link to the user's local copy or a bundled copy? Because finding the user's copy might be difficult (especially on Windows).

[mark-wiemer]

Linking to the bundled copy would be ideal and the only scenario that we'd need to support