Made lexer more efficient

[cfehse]

What has Changed?

• Made the lexer in webview/lexer.ts more efficient. The output for the sample code in the issues will not be printed as quickly as in a console REPL but nearly.
• Changed the regex for the multiline str-inside token type to not allow empty matches. Empty matches can lead to infinite loops if one tries to read all matches for a source in a loop and these empty matches are invalid anyway.
• Added a fallback for the old direct retrieval method in case the lexer position is changed from the outside. This can happen in multiline mode of the clojure lexer in webview/clojure-lexer.ts .

Fixes #128 #228

My Calva PR Checklist

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• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.

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  • Smoke tested the extension as such.
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Ping @PEZ, @kstehn, @bpringe