support for inline codelens

[mickaelistria]

Currently, codelens are only line "headers". In practice, several examples show that such text decoration can also be pretty useful inline, between two characters. For example, IntelliJ (and soon Eclipse IDE) allow to show the parameter name on ambiguous methods.
CodeLens should be allowed to receive as parameter a text position (line/column) where to be inserted, and in case column is missing, it could default to current behaviour which is about making a line header.

[kjeremy]

rust-analyzer has an issue for figuring out how to upstream their version of this.

[angelozerr]

IMHO I think #745 should be more generic than Codelens only.

[dbaeumer]

I will fold this with #956