Add flag for disabling HTML escaping

Adds the DisableEscapeHTML flag for disable the automatic
escaping of the HTML characters '>', '<' and '&'.

The previous commit introduced a potentially breaking change by
removing HTML escaping altogether. This commit fixes that issue
by allowing the user to choose at runtime.

Disable html escaping

This commit removes the automatic escaping of html characters,
effectively rendering JSON strings in the same way as is the
builtin Go encoder with SetEscapeHTML(false).
This should not affect the quality of the resulting JSON and
hopefully will not cause any downstream issues.

Fix backspace and form-feed for Go 1.22

Added Escape function

This commit adds the Escape function for escaping a path
component, making it possible to directly querying keys that have
special characters like dots.

```
json := `{
  "user":{
      "first.name": "Janet",
      "last.name": "Prichard"
    }
}`
user := gjson.Get(json, "user")
println(user.Get(gjson.Escape("first.name")).String())
println(user.Get(gjson.Escape("last.name")).String())
// Output:
// Janet
// Prichard
```

See #333

Mention the dig modifier

Added new tilde types and fixed ~false

This commit fixes an issue with ~false where the it's value was
simply the opposite of ~true. Now ~false explicitly checks for
false-ish values.

Also added ~null and ~* for testing null-ish and non-existent
values.

see #327

Add test

Merge pull request #291 from janisz/master

Handle modifiers with options

Allow for Index > 0 on path compontent that are not modifiers.

This commit fixes an issue where non-modifier path components
such as '@hello' return 0 for the Result.Index value.

Remove encoding/json dependency

The only purpose of using the built-in Go was to encode json
strings that had unicode or needed to escaped.

This commit adds the new function `AppendJSONString` which allows
for appending strings as their json representation to a byte
slice.

It's about 2x faster than using json.Marshal.