Allow variables in snippets

[savetheclocktower]

Have you checked for existing feature requests?

• Completed

Summary

TextMate allows for certain variables to be defined in snippets that map to various things when the snippet is expanded. Most of these have also made it into the LSP specification for snippets.

Pulsar should support this entire list. This means not just substituting them during snippet expansion, but also allowing them to be transformed using the same syntax that already exists for transforming tab stops.

This enhancement does not depend on #8, but for reasons described in that issue, it would be odd to add support for variables without also making snippets triggerable via keystroke.

What benefits does this feature provide?

Aside from the new functionality it enables, it makes our snippets more compatible with the LSP specification, and with snippets as they've been implemented by VSCode, Sublime Text, and TextMate.

Any alternatives?

Not really. If this functionality is worth adding, it's worth doing it this way instead of introducing one that's purposefully incompatible.

Other examples:

See also the VSCode documentation for snippets.

[savetheclocktower]

I plan to tackle this enhancement myself, but probably not until we come to consensus on #8. Until then it's worth capturing in issue form.