update lexicon for latest docs

Closes #2699

Fixes issues with:
* 'string' function synopsis
* Redirection display issues
* Better file & path detection
* Rendering of % & @ chars in both html and man
* @ symbol in tutorial

Improves robustness by implementing an @EOL marker to prevent hold buffer dumping extra chars after the end of an expression.

Added new '{{' and '}}' meta-chars for when you want curly braces in a regexp that was previously tripping up the lexicon.

Improve man/html presentation consistency for
* string
* printf
* prompt_pwd
* type

Use cli-styling for 'practical' examples.

Add <bs> tag for presenting content with preceding backslash.

Signed-off-by: Mark Griffiths <mark@thebespokepixel.com>

doc_src/FORMATTING.md

@@ -160,6 +160,8 @@ The following can be used in \\fish blocks to render some fish scenarios. These
 - `<error>`: \<error\>This would be shown as an error.\</error\>
 - `<asis>`: \<asis\>This test will not be parsed for fish markup.\</asis\>
 - `<outp>`: \<outp\>This would be rendered as command/script output.\</outp\>
+- `<bs>`: Render the contents with a preceding backslash. Useful when presenting output.
+- `{{` and `}}`: Required when wanting curly braces in regular expression example.
 
 ### Prompts and cursors