FEI-4409: Extract i18n strings

[kevinbarabash]

Summary:

Once we move the Perseus code into this repo we'll need to be able to extract strings from it. This PR ports the extract-perseus-strings.js script from webapp for just that purpose.

Since both perseus and perseus-editor will contain some i18n strings, we output one strings.js file per package. The file for perseus contains all of the strings for math-input as well.

TODO:

• add a i18n._() call to perseus/index.js and perseus-editor/index.js to ensure that strings are being extracted from those locations

Issue: FEI-4409

Test plan:

• yarn extract-strings, see that strings have been extracted to dist/strings.js

kevinbarabash@Kevins-MacBook-Pro perseus % yarn extract-strings
yarn run v1.22.17
$ ./utils/extract-strings.js
[INFO ]: wrote 46 strings to /Users/kevinbarabash/khan/perseus/packages/perseus/dist/strings.js
[INFO ]: wrote 1 strings to /Users/kevinbarabash/khan/perseus/packages/perseus-editor/dist/strings.js
✨  Done in 1.50s.

[kevinbarabash]

I'll try turning these back on tomorrow and fixing the actual issue.

[jeremywiebe]

Looks good. Was able to generate the strings on my machine and require them in a node REPL.