view: the code pane's lexer differs from the playground's #2085

Found in the review of #2057, which adds the experimental view. Paths refer to #2059's head.

Problem

internals/view.md says the view's code pane uses the playground's lexical colouring. In fact the view has its own lexer: tokenize in web/view/src/formula.ts, which web/view/src/graphcalLanguage.ts builds on. The playground's is web/playground/src/graphcal-language.ts. The two disagree:

• They treat the reserved word as differently.
• The view's lexer does not accept _ in an exponent; the playground's does.

Proposed fix

• First, correct internals/view.md.
• Then share one lexer between the playground and the view, since they do the same job.