Port FormBuilder from Rails template (#61)

- Add capybara for html rspec matchers
- Port over USWDSFormBuilder to Flex::FormBuilder
- Add memorable_date function to form builder

## Context

I think the form builder makes more sense in the flex module because:
1. We already have a case where a function we want to add to form
builder (memorable_date) relies on Flex functionality (the
memorable_date flex_attribute)
2. Over time it would be nice to add more functionality to the form
builder that extends beyond the basic building blocks from USWDS. For
example we'd want to be able to generate form elements for other
flex_attributes like full_name, address, etc.

This form builder is a straight port of USWDSFormBuilder with the
following changes:
1. Namespaced to Flex module
2. Scoped content strings to flex.*
3. Renamed helper method from us_form_with to flex_form_with

Note: the added capybara gem is for rspec html matchers like
"expect(html_string).to have_element(...)"

Author: lorenyu

Gemfile

@@ -34,4 +34,5 @@ end
 
 group :test do
   gem "simplecov", require: false
+  gem "capybara"
 end

Gemfile.lock

@@ -85,6 +85,15 @@ GEM
     benchmark (0.4.0)
     bigdecimal (3.1.9)
     builder (3.3.0)
+    capybara (3.40.0)
+      addressable
+      matrix
+      mini_mime (>= 0.1.3)
+      nokogiri (~> 1.11)
+      rack (>= 1.6.0)
+      rack-test (>= 0.6.3)
+      regexp_parser (>= 1.5, < 3.0)
+      xpath (~> 3.2)
     coderay (1.1.3)
     concurrent-ruby (1.3.5)
     connection_pool (2.5.0)
@@ -161,6 +170,7 @@ GEM
       net-pop
       net-smtp
     marcel (1.0.4)
+    matrix (0.4.2)
     method_source (1.1.0)
     mini_mime (1.1.5)
     minitest (5.25.5)
@@ -356,6 +366,8 @@ GEM
       base64
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
+    xpath (3.2.0)
+      nokogiri (~> 1.8)
     yard (0.9.37)
     zeitwerk (2.7.2)
 
@@ -371,6 +383,7 @@ PLATFORMS
 
 DEPENDENCIES
   actioncable
+  capybara
   flex!
   guard-rspec
   listen

app/helpers/flex/application_helper.rb

@@ -1,4 +1,8 @@
 module Flex
   module ApplicationHelper
+    def flex_form_with(model: nil, scope: nil, url: nil, format: nil, **options, &block)
+      options[:builder] = Flex::FormBuilder
+      form_with model: model, scope: scope, url: url, format: format, **options, &block
+    end
   end
 end