New setting for `$PATH`

[Taitava]

Discussed in https://github.com/Taitava/obsidian-shellcommands/discussions/150

Develoment:

• [x] The PATH augmentation setting & logic.
• [x] A new variable {{environment:variable_name}}.
• [x] Consider should variables in the path augmentation string be parsed always without escaping special characters. (Edit 2022-05-02: I considered it, and decided that because other environment variables might benefit from variable value escaping, PATH additions should use a similar mechanism, but documentation should advice users to use unescaped variable values by using {{! instead of {{.)
• [x] Add a documentation link to settings.
• [x] Fix a bug that causes all fields to save their edits to the last setting (to Windows Path additions).

Other things:

• [x] Add to CHANGELOG.
• [x] Write documentation (under Environments, which is renamed from Operating systems and shells).
• [x] In documentation, mention that for {{environment:variable_name}}, on Windows the variable_name is case-insensitive, but on macOS and Linux they are case-sensitive. At least I assume so. I have tested on Windows, but will need to test on Linux. I can't test on macOS, but I just assume it's the same as on Linux. Edit 2022-05-01: Tested on Linux, it's case-sensitive.
• [x] Make tests

[Taitava]

I've done some finalising for this feature in the past couple of days: bug fixing and writing documentation. Now I think it's ready and just waiting for a beta 0.13.0 test.

[Taitava]

Added a test and a small addition to the documentation. Now everything should be ready here.

[Taitava]

Released.