Feature: Camel case from/to underscore conversion

[huandu]

Camel case to underscore.

• SomeWords => some_words
• HTTPServer => http_server
• NoHTTPS => no_https

Underscore to camel case.

• some_words => SomeWords
• http_server => HttpServer
• no_https => NoHttps
• _complex__case_ => _Complex_Case_

[huandu]

Add a new rule in camel case to underscore (snake) case.

• Add _ before a number if there is a character other than a number, blank, underscore or hyphen.

Some cases.

• HTTP2XX => http_2xx
• http2xx => http_2xx
• HTTP20xOK => http_20x_ok

[huandu]

As snake/kebab case conversion is aware of space, ToCamelCase should be aware of it too.

To make the conversion rule clear, we define it as following.

Camel/snake/kebab case conversion is to convert words separated by space, underscore and hyphen to expected case. For snake or kebab case conversion, when an upper case rune is converted to lower case, a new underscore or hyphen is inserted before the rune.

For the string FROM CamelCase to snake/kebab-case, following are the results after conversion.

• Camel case: FromCamelcaseToSnake/kebabCase.
• Snake case: from_camel_case_to_snake/kebab_case.
• Kebab case: from-camel-case-to-snake/kebab-case.

[huandu]

Add a new rule to make string starting with number more readable.

If there are lower case characters immediately following numbers, e.g. 20x in HTTP20x or 2m3s in Duration2m3s, all these numbers and lower case characters will be considered as one word during conversion. Otherwise, numbers will be considered as a part of previous word, e.g. Bld4 in Bld4Floor3rd.

Following are some samples.

• HTTP20x => http_20x
• Duration2m3s => duration_2m3s
• Bld4Floor3rd => bld4_floor_3rd

[huandu]

BREAKING CHANGE

Per discussion in #56 and the history around PascalCase and CamelCase, ToCamelCase should return first letter in lowercase. Instead, add a new api ToPascalCase to cover the old test cases.

For the string FROM CamelCase to snake/kebab-case, following are the results after conversion.

• Camel case: fromCamelcaseToSnake/kebabCase.
• Pascal case: FromCamelcaseToSnake/kebabCase.
• Snake case: from_camel_case_to_snake/kebab_case.
• Kebab case: from-camel-case-to-snake/kebab-case.