fix: handle template literals in startsWith

[jly36963]

Relates to: #181

Description

I needed to have more granular control over the comparison, so I replaced the
${S}${string}
part with
${infer THead}${infer TRest}

I needed to remove Length<T> in a lot of places -- it doesn't work with template literals.

Details

Length does not (and cannot) handle string interpolation well.

Length<'abc'> // 3
Length<`abc${string}`> // number

Slice<T, startIndex, endIndex> uses Length<T> as the default value for endIndex.
Both StartsWith/EndsWith use Slice.

I refactored Slice/StartsWith/EndsWith to not use Length (or only use it when T is known to be a literal) and I now have them working with template literals.

Also, slice/startsWith/endsWith are not methods with default number arguments (eg: 0 or Length<T>), they are overloads where the last arguments are optionally provided.

Consider

slice(start?: number | undefined, end?: number | undefined): string
startsWith(searchString: string, position?: number | undefined): boolean
endsWith(searchString: string, endPosition?: number | undefined): boolean

In other words, updating the params for these types to be
P: number | undefined = undefined
instead of
P: number = Length<T>
makes this fix possible and more closely aligns our types with the TS definitions for those string methods.

[gustavoguichard]

All good to me! The code is more complex than what I wanted but I trust you took the right decisions!
TY very much for the PR!!