Default values for variables

[Taitava]

Discussed in https://github.com/Taitava/obsidian-shellcommands/discussions/146

• [x] Default values: if a custom variable does not yet have a value, a default value for it can be defined in each shell command's settings. Default values are shell command specific, so two shell commands can define different default values for the same variable.

• [x] In addition to default values, add an option to silently cancel execution if the variable does not have a value. Maybe not so useful for custom variables, but will be good for built-in {{file_*}} variables when executing shell commands via events in a situation where no file is active, in which case the execution can be automatically cancelled without error messages.

• [x] Parse {{variables}} in default values.

I decided to create a separate issue for this feature in order to make it a bit more prominent, not just someting that happens to come along with other features. So it will also have a record in CHANGELOG.md. Being able to supress {{file_*}} variables' availability errors can be so handy in some cases (e.g. events) that it deserves a bit more attention.

[Taitava]

Beta 0.12.0-beta.1 was released yesterday. Following commits will go into a second beta version.

Edit 2022-05-07: No second beta was made, the commits are included in the normal 0.12.0 release.

[Taitava]

Released.