Refactor BufferLocalStore related code

[Martin521]

Description

This PR comprises two commits.

The first one is pure refactoring.
It extracts all the code related to LexBuffer.BufferLocalStore from ParseHelpers.fs and moves it to a separate file LexerStore.fs.
Some common code is factored into separate functions (at the top of the file). Some names are simplified.
In consuming code, open statements and names are adapted accordingly.
This refactoring, I believe, makes the use of BufferLocalStore easier to understand, track and maintain.
The separation into LexerStore.fs makes also sense because all the code is similar in that it collects information during lexing for later "meta" use (like editor support, compiler configuration, documentation.), and there is no direct relation to the rest of the code in ParseHelpers.fs.
It is part of my work on "scoped nowarn" (that will also need to make use of BufferLocalStore), but separated out.

The second commit removes resetting the store in LexHelp.reusingLexbufForParsing.
I found that all code paths leading to this place come with a freshly created Lexbuf, i.e. empty BufferLocalStore.

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[psfinaki]

Thank you Martin!