REPL: default to preset-env that matches babel version

Some followup to https://github.com/babel/website/pull/1415.

If I load the repl with the following (no presets enabled, babel@7.0.0-beta.38):
http://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&code_lz=MYGwhgzhAEBCYCMCmJoG8C-Q&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=true&lineWrap=true&presets=&prettier=false&targets=&version=7.0.0-beta.38

And then click preset-env, it loads preset-env@7.0.0-beta.32.

Another example, (preset to env, babel@7.0.0-beta.38, no env version stated):
http://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&code_lz=MYGwhgzhAEBCYCMCmJoG8C-Q&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=true&lineWrap=true&presets=env&prettier=false&targets=&version=7.0.0-beta.38

In both of these cases, I think it should load a matching preset-env version (if possible)?

Also, right now we're always writing the env version to the query string... I feel like we should support it without.

/cc: @yavorsky

Start with the Babel website REPL entry point and inspect how the version and preset query parameters select preset-env. Compare the loaded preset-env version with the Babel version, including URLs where the env version is omitted. Done means the REPL selects a matching preset-env version when possible and supports omitting that version from the query string.

Written by the indexing model from the issue text.