Make snippet parsing and expansion lazy

Requirements

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All new code requires tests to ensure against regressions

Description of the Change

Defers parsing of snippet bodies until used for an expansion. Also defers calculation of the initial expansion text and tab stops until used.

This lazy loading is beneficial, as the majority of snippets will not be used and therefore do not need to be parsed. The parse tree is also cached on the snippet, so there is no cost for subsequent uses.

Deferring the expansion text calculation is less useful, but necessary to make way for the introduction of variables, which may change between uses (e.g., a timestamp variable). However I've included detection of variables, and if not present the expansion details are also cached.

Some redundant properties have also been removed, such as snippet on SnippetExpansion because it's uses could be replaced with local variables or more specific properties of the snippet.

No tests have been added (should lazy loading / caching be tested? The visible part should be the same as previous, and that's already got tests). A test for caching at the Snippets level has been removed, as it is now done on the Snippet itself. There is some existing support for lazy Snippet creation, so I can probably remove that too now that creating a Snippet instance is trivial.

Alternate Designs

In order to support variables, some kind of processing on use is necessary.

Benefits

Reduced memory footprint (about 3x smaller when no snippets used).
Gives room to support dynamic features like variables

Possible Drawbacks

Some properties have changed, and I don't think this package has a well defined public interface. E.g., the settings-view package accesses snippet fields directly. It doesn't appear to be affected by this change though, as the bodyText field is kept (though could theoretically be removed once the parse tree is made).

Applicable Issues

Broken tests unrelated; see #307

Progress for #288

atom / snippets