followup strformat PR. backslash escapes, tests, docs (#17700)

* Allow use of colons inside fmt
allowing colons inside fmt by replacing the format specifier delimiter lets arbitrary nim code be run within fmt expressions.

Co-authored-by: flywind <xzsflywind@gmail.com>

* formatting,documentation,backslash escapes

Adding support for evaluating expressions by special-casing parentheses causes this regression: `&"""{ "(hello)" }"""` no longer parses.
In addition, code such as &"""{(if open: '(' else: ')')}""" wouldn't work.
To enable that, as well as the use of, e.g. Table constructors inside curlies, I've added backslash escapes.
This also means that if/for/etc statements, unparenthesized, will work, if the colons are escaped, but i've left that under-documented.

It's not exactly elegant having two types of escape, but I believe it's the least bad option.

* changelog
* added json strformat test
* pulled my thumb out and wrote a parser

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: flywind <xzsflywind@gmail.com>

changelogs/changelog_X_XX_X.md

@@ -13,6 +13,7 @@ The changes should go to changelog.md!
 
 - Changed `example.foo` to take additional `bar` parameter.
 
+- Added support for evaluating parenthesised expressions in strformat
 
 ## Language changes