[Refactor] Stop rerendering directives, inject imports instead

[ckwalsh]

Summary:
This commit changes how the sorted imports are combined with the
original source.

Prior to this commit, all ImportDeclaration nodes and their leading
comments, plus any InterpreterDirective and Directive nodes, were
extracted from the original code and re-rendered using babel. The
rendered nodes were then concatenated with the original source with
those nodes removed to produce the updated source.

This approach safely protected against functional changes, but
removed newlines around comments near the beginning of the file when
the first node of the original source was an ImportDeclaration, as
babel does not preserve whitespace when rendering content.

If a user has configured a plugin that attempts to manage comments
and/or whitespace near the top of the file, such as auto-inserting a
license header (as I am trying to do), this results in conflicts /
formatting churn.

This commit does not directly resolve this incompatibility, however
it better prepares the codebase for a plugin option to be added that
can resolve the issue.

Test Plan:
yarn install && yarn run test --all

Note that one snapshot was changed by this commit where a newline was
changed, acting as an effective example of how the original approach
could affect whitespace in the re-rendered portion of the file.

Stack created with Sapling. Best reviewed with ReviewStack.

• [Feature] importOrderIgnoreHeaderComments option #359
• [Feature] Expand detection for sort-imports-ignore #358
• -> [Refactor] Stop rerendering directives, inject imports instead #357
• [Perf] Improve performance of removeNodesFromOriginalCode() #356
• [Test] Stop ignoring exceptions thrown in snapshot tests #355
• [Format] Run prettier on most files #354