Add pathRewrite to proxy options

[pete-murphy]

Problem

What issue is this causing for Elm Land users?

Fixes #176

Solution

How does this code change address the problem described above?

A user can specify a pathRewrite object in a proxy definition

    "proxy": {
      "/api": {
        "target": "http://localhost:3000",
        "pathRewrite": { "^/api": "" }
      },
    }

to proxy requests from "/api" to "http://localhost:3000".

The behavior is based on the pathRewrite option in http-proxy-middleware where the first matching pattern determines the rewrite: https://github.com/chimurai/http-proxy-middleware/blob/f2a0af329969f8a70b3967a969146758a73d8b0e/src/path-rewriter.ts#L33-L39

Notes

(Can be blank!) Are there any unrelated code changes or other notes you'd like to share regarding this PR?

I tested this locally by running elm-land server with proxy options supported by the existing implementation, and again with the new pathRewrite option, and proxying behaves as expected in both cases. I don't know how to add tests for this 😅 I'd be open to suggestions.

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