MDL-52136 core: Add support for quoting variables in mustache helpers

MDL-52136 core: Add support for quoting variables in mustache helpers

This is required for when helpers include json-encoded variables as arguments.
As an example, imagine a template with content:

{{# str }} somekey, someidentifier, { "fullname": "{{ fullname }}" } {{/ str }}

If the fullname variable were to include the double-quote character (e.g.
John "Trevor" Doe) because of the way in which mustache renders content, it
would become:

{{# str }} somekey, someidentifier, { "fullname": "John "Trevor" Doe" } {{/ str }}

This results in an invalid JSON structure.

To work around this issue, the quote characters in the passed variable
must be escaped:

{{# str }} somekey, someidentifier, { "fullname": "John \"Trevor\" Doe" } {{/ str }}

Unfortunately, Mustache provides no way of doing so natively.

With this function, we can quote the text as appropriate:

{{# str }} somekey, someidentifier, { "fullname": {{# quote }}{{ fullname }}{{/ quote }} } {{/ str }}

This also handles the case where the quoted content includes the Mustache
delimeter ({{ or }}).

For example:
fullname = 'John "}}Trevor{{" Doe'

Ordinarily this would be rendered as:
{{# str }} somekey, someidentifier, { "fullname": "John "}}Trevor{{" Doe" } {{/ str }}

This rendering is both a JSON error, and also a mustache syntax error because of the mustache delimeters.

The quote helper also escapes these by wrapping them in change delimeter
tags:

{{# str }} somekey, someidentifier, { "fullname": "John "{{=<% %>=}}}}<%={{ }}=%>Trevor{{=<% %>=}}{{{{=<% %>=}}" Doe" } {{/ str }}