move gdscript.js to root directory.

[fengjiongmax]

Hello, I'm trying to make highlight.dart support GDScript, which has the language definition files generated from highlight.js.
However, unlike highlightjs-vue or highlightjs-graphql have the language definition files in the root directory, gdscript.js located in src/languages.
And I think it's also confusing having package.json's main value set to gdscript.js where there's no such file in the root directory.

[joshgoebel]

The main key should be updated, src/languages is the correct location by convention.

[fengjiongmax]

Following @joshgoebel 's instruction, I've created a PR.
And GDScript syntax definition file for highlight.dart can be generated from this repository, thanks.