[KhadeerBasha1232]

Add template parameters to JsCode

Closes #2121

JsCode now accepts keyword arguments that fill %(name)s placeholders in the code, as proposed in the issue:

handler = JsCode("""
    var treeControl = %(tree)s
    // do something with the control
""", tree=tree_control)

How values are inserted

• A folium/branca Element is replaced by its Javascript variable name (get_name()).
• A JsCode object is replaced by its code, so snippets can be nested.
• Any other value is formatted with the % operator as usual.

Details

• Placeholders are filled in when the code is rendered, not when JsCode is created. So an element's name is read at render time.
• Without parameters, the code is used as is. Existing code with a literal % (e.g. a % b) keeps working. When parameters are given, a literal % must be written as %%, as with normal % formatting.
• js_code is still a readable (and settable) attribute, so templates that use {{ this.handler.js_code }} and tojavascript work unchanged.
• JsCode(other_js_code, **params) keeps the other object's parameters and merges in the new ones.

Changes

• folium/utilities.py: parameters and docstring for JsCode.
• tests/test_utilities.py: tests for elements, nested JsCode, plain values, %% escaping, no-parameter passthrough, merging, render-time resolution, and rendering inside a map event handler.
• docs/advanced_guide/customize_javascript_and_css.md: new section "Referring to other map objects in JsCode".