fix(gatsby-plugin-typescript): use TS parser for ts files and replace some vanilla js rules with typescript ones

[pieh]

This makes sure we use TS parser for .ts(x) files in built-in linting with webpack. I also adjust few rules from vanilla js to typescript ones (like no-unused-vars is needed because vanilla js will show warnings for type only imports)

Fixes #21913

[blainekasten]

This code looks good but for some reason I'm not really able to get my local set up to test these changes so I can't be confident to approve.

[pieh]

Ok, so babel-preset-react-app does have overrides for .ts(x)? files. So my changes are somehow working around other issues (apparently in eslint or in our config). This is wrong fix, so I'll close it