Upgrade Babel to 7.10.1 from 7.0.0.

Unit tests started failing in Travis (despite the fact that
this repo hasn't been modified in more than a month) with
errors like the following from
https://travis-ci.org/github/derat/ascenso/builds/692361530:

[BABEL] /home/travis/build/derat/ascenso/src/views/Profile.test.ts:
No "exports" main resolved in /home/travis/build/derat/ascenso/node_modules/@babel/helper-compilation-targets/package.json
(While processing: "/home/travis/build/derat/ascenso/node_modules/@vue/cli-plugin-babel/preset.js")

Some searching points toward
babel/babel#11216, alleging that
this is caused by a regression (or maybe just a change in
undocumented behavior) in Node 13.9. Babel 7.8.7 and later
apparently contains a workaround.

The Travis logs claim that Node 12.17.0 is still being used,
so I have no idea why failures are occurring there. I don't
see any other clues, though, so I'm giving this a shot.

Also switch from the unscoped babel-core NPM package to the
scoped @babel/core package. It looks like the unscoped
package is no longer getting updated.

This only explictly upgrades @babel/core and
@babel/helper-compilation-targets. It looks like there are a
bunch of other packages in the @babel scope still stuck at
7.8.3, but I have no idea how to convince NPM to upgrade
those.

Author: derat