Move `ArcanistSpellingDefaultData` into a configurable JSON file

Summary: Currently, the `ArcanistSpellingLinter` loads data from `ArcanistSpellingDefaultData`, with no way to configure the linter from an `.arclint` file. Instead we should define a format for a "dictionary" file, of which the `ArvcanistSpellingLinter` can load and of which the paths are easily configured through `.arclint`.

Test Plan:
Updated the test case and ran `arc unit`.

NOTE: I have removed the `LINT_SPELLING_PICKY` and `LINT_SPELLING_IMPORTANT` constants and replaced them with `LINT_SPELLING_FULL` and `LINT_SPELLING_PARTIAL`. This was done because it simplifies the implementation considerably and makes customization of the `ArcanistSpellingLinter` simpler, but also because these constants were not widely used in the existing implementation.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9805