SpEL: add content-assist for bean identifiers in SpEL expressions

[martinlippert]

With the recent addition of Go To Definition for bean identifiers in SpEL expressions, we could also provide content-assist for bean identifiers in SpEL expressions as one of the next steps here.

While figuring out proposals is easy (we have this for many annotations already), providing a smooth typing experience in those SpEL expressions needs some investigation here, especially around how well the parser deals with incomplete SpEL expressions and how we can find the best content-assist proposals in those situations.