[gatsby-source-filesystem] Ignore Bower components directory (#8830)

This is a very small change to the `gatsby-source-filesystem` plugin to ignore `bower_components` directories by default. This is to avoid creating filesystem nodes for any dependencies installed with Bower. Although Bower usage is falling it is still in widespread use in older projects.

packages/gatsby-source-filesystem/src/gatsby-node.js

@@ -134,6 +134,7 @@ See docs here - https://www.gatsbyjs.org/packages/gatsby-source-filesystem/
       `**/.npmignore`,
       `**/.babelrc`,
       `**/yarn.lock`,
+      `**/bower_components`,
       `**/node_modules`,
       `../**/dist/**`,
       ...(pluginOptions.ignore || []),