[EuiCodeBlock] Improve syntax highlighting for `language="esql"`

**Requestor**

@lucaslopezf

**Problem Statement**

The syntax highlighting for the `esql` language in `EuiCodeBlock` is different from the one in the Monaco editor.

(Monaco editor on the left, `EuiCodeBlock` on the right)

**Proposed Solution**

Improve the styles of the `esql` language in `EuiCodeBlock` to get close to a 1:1 match with the Monaco editor.

**Use Case**

Improve user experience.

**Value / Impact**

The end-user benefits

**Do alternatives or workarounds exist?**

No.

**Additional context (Optional)**

The PR that originated the request: https://github.com/elastic/kibana/pull/254537