fix($getFilterMethod): prevent camelCase from stripping the leading $ of filter name

[zaosoula]

Hi,

I was trying to implement special filters starting with a $ chat ($select, $sort). And while testing i encountered an issue, $getFilterMethod strip the leading $ of the filter name while calling lodash/camelCase. This PR fixes it and allow the use of $filters

[zaosoula]

Our use-case as an example

import { BaseModelFilter } from 'adonis-lucid-filter'

export default class CommonFilter<T> extends BaseModelFilter {
  $sort(value: Record<keyof T, 'asc' | 'desc'>): void {
    if (typeof value === 'object') {
      Object.entries(value).forEach(([key, direction]) => {
        this.$query.orderBy(key, direction)
      })
    }
  }

  $select(value: string | string[]): void {
    this.$query.select(Array.from(new Set(['id', ...(Array.isArray(value) ? value : [value])])))
  }

  $populate(value: string | string[]): void {
    ;(Array.isArray(value) ? value : [value]).forEach((relation) => {
      this.$query.preload(relation)
    })
  }
}