[snippets] Add text block replacements

[david-caro]

It would be nice to have some snippets to help refactor class-based tests to dsl-based tests, using text block selection.

From #223:

It may make sense to use the TM_SELECTED_TEXT fragment in things like the snippet for creating an example. Someone might be porting an existing block of code, and if they've selected some text - Ctrl-Shift-P, Insert Snippet, arrow key to relevant snippet - will let them just wrap the selection. It takes a default, so you can do ${TM_SELECTED_TEXT:default_if_no_selection}.