Fix: Insert centring container around the yield

[dixpac]

Insertion point of the centering container was problematic.
Meaning, if someone has a yield :somethig
inside theirs application.html.erb old regex would pick that as a
insertion point and possiblly break the html structure.

This commit makes that regex more rigid to only insert centering
container around the <%= yield %>.

Fixes: #98

[dixpac]

One more thing we could do here is to ask a question does user want to insert centering contianer

if yes?("Do you want to insert self-centering html attribute inside your aplication.html.erb")
  insert_into_file APPLICATION_LAYOUT_PATH.to_s, %(    <main class="container mx-auto mt-28 px-5 flex">\n  ), before: /^\s*<%= yield %>/
  insert_into_file APPLICATION_LAYOUT_PATH.to_s, %(\n    </main>),  after: /^\s*<%= yield %>/
end

This would only be useful for folks installing tailwindcss-rails on already established app UI aesthetics

[dixpac]

cc @dhh (not sure if you forgot about this patch, or you don't like the solution 😃 )

[dhh]

All good 👍