[streams] Style guide (#421)

I'd like to have a memorandum about the rules we follow when writing this spec.

Here's a draft.

## Spec text

- Put an <a> tag only on the first occurrence of the term in a paragraph
  - https://github.com/whatwg/streams/pull/417#discussion_r48513698
- Use "set" for updating internal slots
- Use "let" for defining local variables
- Add `nothrow` or `throw` attribute to the headings tag of abstract operations to annotate them about exception throwing

## Reference implementation

- Wrap long lines to fit 120 col
- Alphabetize imports
  - https://github.com/whatwg/streams/pull/277#discussion_r24117515


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/421