Code span highlighting

[andrewbranch]

Closes #76

Copying implementation instructions from issue:

• Make the changes to types.d.ts I mentioned above.
• Introduce a new type into schema.graphql to represent inline code. It should look just like GRVSCCodeBlock, bet can replace preClassName and codeClassName with just className, omit meta, and replace tokenizedLines with tokens: [GRVSCToken!]!. (Run npm run build after this to update schema.d.ts.)
• Add validation for “if options.inlineCode exists, options.inlineCode.marker must exist in validateOptions.js.
• Update the AST visitor to look for both code and inlineCode nodes, if options.inlineCode is defined. (I think you can replace the string 'code' test with a callback function that accepts a node, where you can check its type. API)
• In the initial loop over code nodes, you’ll need to branch on node.type for some things. E.g., instead of calling parseCodeFenceHeader, you’ll need to call a new similar parsing function that separates the node text into language and text. If you don’t detect a language (i.e., you never see inlineCode.marker), I think you can just continue and be done with that node. The call to getPossibleThemes needs to use inlineCode.theme || theme if node.type === 'inlineCode'. Otherwise, I think most of that loop body stays the same. In the registerCodeBlock function, I think we should probably not run line transformers on inline code nodes.
• The next loop, registry.forEachCodeBlock, goes back over the nodes and generates HTML for them. The actual HTML generation is in getCodeBlockDataFromRegistry. Probably about half the stuff in here is actually specific to code blocks, and I think it might be too messy to make it polymorphic, so I would probably recommend making registry.forEachInlineCode and getInlineCodeDataFromRegistry functions instead, and repeating/extracting the relevant things from what’s there now. While generating HTML is the bulk of the work in this function, its full purpose is to generate a JavaScript object matching the new type you added to the GraphQL schema in step 2.
• Back in index.js in your [codeBlockRegistry.forEachInlineCode] loop, you can finish generating the GraphQL node in the same way as registry.forEachCodeBlock and push it onto the same graphQLNodes array.
• Write some integration tests. Try to cover the API surface.

And a step I forgot:

• Add a new GraphQL resolver for GRVSCCodeSpan in gatsby-config.js