Add support for variable operator syntax

[Mpdreamz]

This PR adds syntax support for mutation the contents of variables through a chain of operators.

{{variable | op1 | op2 | op3}}

Operator	Purpose
lc	LowerCase,
uc	UpperCase,
tc	TitleCase, capitalizes all words,
c	Capitalize the first letter,
kc	Convert to KebabCase,
sc	Convert to SnakeCase,
cc	Convert to CamelCase,
pc	Convert to PascalCase,
trim	Trim common non word characters from start and end

For variables declaring a semantic version or Major.Minor the following operations are also exposed

Operator	Purpose
M	Display only the major component
M.x	Display major component followed by '.x'
M.M	Display only the major and the minor
M+1	The next major version
M.M+1	The next minor version

This allows a more generic yet rigorous approach to displaying version numbers in the plethora of different formats the docs might need them to:

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/substitutions.md
• docs/syntax/version-variables.md

[reakaleek]

nice