This function should take three strings as input: the main text, the target substring, and the replacement substring. It should return a new string where all occurrences of the target substring within the main text are replaced with the replacement substring.

Input: Three strings (str).

Output: String (str).

Examples:

assert replace_all("hello world", "world", "TypeScript") == "hello TypeScript"
assert (
    replace_all("hello world world", "world", "TypeScript")
    == "hello TypeScript TypeScript"
)
assert replace_all("TypeScript is fun", "fun", "awesome") == "TypeScript is awesome"
assert replace_all("aaa", "a", "b") == "bbb"

How it’s used:

• in text editors for find-and-replace functionality;
• in data transformation tools to standardize or clean data;
• in website template systems to replace placeholders with actual content.

Preconditions:

• all inputs should be strings: mainText, target, replacement ∈ string;
• the target should not be an empty string: length(target) > 0.